MP_L/P_L = MP_K/P_K for choosing the cheapest input mixDemand for a factor of production (labor, land, capital) is derived demand — it exists only because the factor produces sellable output. A café doesn't want baristas for their own sake; it wants the lattes they make. If latte demand booms, barista demand follows. Every factor-market question on the CLEP exam (Factor Markets is 6–12% of the test) runs on this link: the product market drives the factor market.
Suppose you run a small drink shop and pay $16/hour. Should you hire a fourth worker? You ask one question: will this worker bring in more than $16/hour of extra revenue?
MRP = MP × MR — the extra revenue from hiring one more unit. For a firm selling in a perfectly competitive product market, MR = P, so MRP = MP × P.Hiring rule: employ factor units up to where MRP = MFC (for a wage-taking firm, MRP = W).
| Workers | Output | MP | MRP (P = $2) | Hire at W = $16? |
|---|---|---|---|---|
| 1 | 20 | 20 | $40 | ✓ |
| 2 | 38 | 18 | $36 | ✓ |
| 3 | 52 | 14 | $28 | ✓ |
| 4 | 64 | 12 | $24 | ✓ |
| 5 | 71 | 7 | $14 | ✗ stop at 4 |
The firm's demand curve for labor IS its MRP curve — downward-sloping because of diminishing marginal product (and, for an imperfectly competitive seller, also because MR falls with output).
Trap alert: MRP = MP × the product's marginal revenue — never MP × the wage. The wage is what you compare MRP to.
[GRAPH: Two panels. LEFT (Labor market): downward-sloping market labor demand D_L (the sum of firms' MRPs), upward-sloping market labor supply S_L, equilibrium wage W = $16, market employment L. RIGHT (Individual firm): horizontal line at W labeled "S_L = MFC = W" (perfectly elastic labor supply to the firm); downward-sloping MRP curve labeled "D_L = MRP"; the firm hires l where MRP = MFC.]
The market sets the wage; the individual firm is a wage taker facing a horizontal labor supply curve at W*. This mirrors the perfectly competitive product market, with the horizontal line now representing a cost (the wage) rather than a price received.
Shifters: - Labor demand (MRP) shifts when: the product's price or demand changes, worker productivity changes (training, technology, more capital per worker), or the number of employers changes. - Labor supply shifts when: population or immigration changes, worker preferences change, or opportunities in alternative occupations change. - A change in the wage itself = movement along both curves, never a shift. Same grammar as ordinary supply and demand.
When a firm chooses between labor and capital, cost is minimized where the last dollar spent on each input yields the same marginal product:
MP_L / P_L = MP_K / P_K
Same logic as the consumer's utility-maximizing rule, with MP replacing MU. If MP_L/P_L > MP_K/P_K, labor delivers more output per dollar — shift spending toward labor (diminishing returns then lowers MP_L and raises MP_K until the ratios equalize).
Worked example (a favorite CLEP setup): A firm produces 1,000 units using labor and capital. The last worker adds 40 units and costs $5; the last machine adds 90 units and rents for $15. Per dollar: labor gives 40/5 = 8 units, capital gives 90/15 = 6. Not minimized — use more labor and less capital. Note the trap: raw marginal products (90 > 40) point the wrong way. Always compare per dollar.
Keep the two rules separate: the least-cost rule picks the input mix for a given output; MRP = MFC picks the profit-maximizing level of each input.
A monopsony is a labor market with a single dominant employer — the only hospital for 100 miles, the mining town's one mine. It faces the whole upward-sloping market labor supply: to attract one more worker it must raise the wage, and pay that higher wage to everyone already on staff. So the marginal cost of an extra worker exceeds that worker's own wage — MFC lies above the labor supply curve.
[GRAPH: Monopsony. X-axis "Labor", Y-axis "Wage". Upward-sloping S_L; a steeper MFC curve above it; downward-sloping MRP. Employment Lm where MRP = MFC; wage Wm read DOWN from Lm to the SUPPLY curve — below the competitive wage Wc at Lc (where MRP crosses S_L).]
Monopsony two-step: (1) hire where MRP = MFC → Lm; (2) pay the wage from the supply curve at Lm → Wm. Compared with a competitive labor market: fewer workers (Lm < Lc), a lower wage (Wm < Wc), and W < MRP — workers are paid less than the revenue the marginal worker generates. One noteworthy consequence: a minimum wage set between Wm and Wc can raise both wages and employment under monopsony, unlike in a competitive labor market.
Employers think in MRP terms whether they use the vocabulary or not. The strongest raise argument is not seniority, effort, or what colleagues earn — it is evidence that the revenue your work adds exceeds what you cost: sales you closed, billable hours, clients retained, costs you eliminated. You are showing your employer that at your current wage, MRP > W — hiring (and keeping, and paying more for) you is profitable.
Q1 — E. Factors are wanted only for what they produce, so demand for warehouse labor passes through from demand for the goods being handled. A and D are policies that affect the market but are not the source of the demand itself. B concerns bargaining over the wage, not why the labor is demanded. C describes labor supply, the other curve entirely. Fix rule: derived demand = follow the product — factor demand always traces back to demand for the output.
Q2 — D. MRP is the extra revenue from one more worker: the extra output (MP) times the revenue per unit of output (MR). A is the total wage bill, a cost concept. B is marginal factor cost, the thing MRP gets compared to. C is average product, and averages never drive marginal decisions. E multiplies by the input's price — the classic MRP trap; the wage is the benchmark, not an ingredient. Fix rule: MRP = MP × MR (product side); the wage only enters when you compare.
Q3 — C. MRP per worker: 20×$4=$80, 15×$4=$60, 12×$4=$48, 10×$4=$40, 6×$4=$24. Hire while MRP ≥ W = $48: the 3rd worker's MRP exactly equals $48; the 4th's $40 falls short. A and B stop while MRP still exceeds the wage, leaving profitable hires on the table. D hires a worker who adds $40 but costs $48. E hires everyone available regardless of the rule. Fix rule: build the MRP column (MP × P), then hire every worker whose MRP ≥ wage — stop at the first one below.
Q4 — C. Higher productivity raises MP, hence MRP, at every employment level — labor demand (the MRP curve) shifts right. A shifts labor demand left (lower product price lowers MRP). B is a movement along the demand curve, not a shift. D shifts labor supply right. E is a price control that changes the quantity hired along existing curves; it does not shift demand. Fix rule: labor demand shifts only when MRP changes — product price, productivity, or technology; the wage itself only moves you along.
Q5 — A. A wage taker can hire any number of workers at the going market wage, so the labor supply it faces is horizontal at W, and MFC = W. B describes the supply curve facing a monopsonist, not a competitive hirer. C would mean no hiring flexibility at any wage, which contradicts a functioning labor market. D confuses the labor market with the product market. E confuses the firm's labor supply with its labor demand (MRP). Fix rule: competitive hiring → horizontal labor supply at the market wage; upward-sloping supply to the firm signals monopsony.
Q6 — D. Cleaners' MRP = MP × P doubles at every quantity, so labor demand shifts right, raising the equilibrium wage and employment together. A puts the change on the wrong curve — nothing happened to workers' willingness to supply labor. B misses that a product-price change is exactly what shifts the MRP curve. C wrongly imports product-market logic (higher price → less quantity demanded) into the factor market's shift analysis. E is impossible for a pure rightward demand shift along an upward supply curve. Fix rule: product price up → MRP up → factor demand right → wage and employment both rise.
Q7 — A. Compare output per dollar: labor 40/5 = 8, capital 90/15 = 6. Labor delivers more output per dollar, so substitute toward labor. B follows raw marginal products (90 > 40) — the trap this question is built on. C would require the per-dollar ratios to be equal; 8 ≠ 6. D expands output rather than producing the same output more cheaply. E confuses the least-cost rule with the MRP hiring rule — the input mix decision needs only MPs and input prices. Fix rule: least cost = equal MP per dollar; always divide by the input's price before comparing.
Q8 — D. Equal marginal product per dollar spent is exactly the least-cost condition — no reshuffling of inputs can make the current output cheaper. A and B would break the equality and raise cost. C overreaches: least cost says this output is produced cheaply, not that this output level maximizes profit (that requires MRP = MFC for each input). E confuses equalizing marginal products with equalizing marginal products per dollar. Fix rule: MP_L/P_L = MP_K/P_K = least-cost mix; profit maximization is a separate, additional condition.
Q9 — E. Employers hire and retain when MRP ≥ wage, so demonstrating that her work generates more revenue than her pay costs speaks directly to the employer's decision rule. A appeals to seniority, which affects MRP only if experience raises productivity. B cites totals, but hiring logic runs on margins — high totals can coexist with a wage above MRP. C describes her costs, not the firm's benefits; it does not change her MRP. D is an equity comparison that says nothing about whether her MRP covers her wage. Fix rule: the wage case an employer must respect is MRP > W — show revenue added, not tenure, need, or comparisons.
Q10 — B. A monopsonist faces the upward-sloping market supply curve, so each additional hire requires a higher wage paid to the entire staff — total labor cost jumps by more than the new nurse's wage. A describes competition from other employers, the opposite of monopsony. C is true of MRP but explains the demand side, not why MFC exceeds the wage. D confuses product-market monopoly with factor-market monopsony — they are independent. E describes a competitive hirer, where MFC would equal the wage. Fix rule: monopsony MFC > W because the raise for incumbents rides along with every new hire.
Q11 — B. The monopsonist hires where MRP = MFC, which is fewer workers than where MRP crosses supply, and then pays the lower supply-curve wage at that employment level. A and D wrongly assume market power lets the buyer of labor pay more. C cannot happen: restricted hiring and a wage read lower on the supply curve go together. E misses that the wage falls below the competitive level, not just employment. Fix rule: monopsony = fewer workers and a lower wage — quantity from MRP = MFC, wage from the supply curve below.
Q12 — C. The wage is the price on the labor market's own axis; when it changes, the firm slides along its MRP (labor demand) curve, hiring fewer baristas at the higher wage — but the curve itself has not moved. A misapplies product-market shift logic; input prices don't shift the input's own demand curve. B is false: MRP depends on MP and the product's price, not on the wage. D commits the same shift error in the opposite direction. E moves the mistake to the other curve — a wage change doesn't shift labor supply either; it moves along both curves. Fix rule: a factor's own price never shifts that factor's demand or supply — own-price changes are movements along.
Q1 — E. Factors are wanted only for what they produce, so demand for warehouse labor passes through from demand for the goods being handled. A and D are policies that affect the market but are not the source of the demand itself. B concerns bargaining over the wage, not why the labor is demanded. C describes labor supply, the other curve entirely. Fix rule: derived demand = follow the product — factor demand always traces back to demand for the output.
Q2 — D. MRP is the extra revenue from one more worker: the extra output (MP) times the revenue per unit of output (MR). A is the total wage bill, a cost concept. B is marginal factor cost, the thing MRP gets compared to. C is average product, and averages never drive marginal decisions. E multiplies by the input's price — the classic MRP trap; the wage is the benchmark, not an ingredient. Fix rule: MRP = MP × MR (product side); the wage only enters when you compare.
Q3 — C. MRP per worker: 20×$4=$80, 15×$4=$60, 12×$4=$48, 10×$4=$40, 6×$4=$24. Hire while MRP ≥ W = $48: the 3rd worker's MRP exactly equals $48; the 4th's $40 falls short. A and B stop while MRP still exceeds the wage, leaving profitable hires on the table. D hires a worker who adds $40 but costs $48. E hires everyone available regardless of the rule. Fix rule: build the MRP column (MP × P), then hire every worker whose MRP ≥ wage — stop at the first one below.
Q4 — C. Higher productivity raises MP, hence MRP, at every employment level — labor demand (the MRP curve) shifts right. A shifts labor demand left (lower product price lowers MRP). B is a movement along the demand curve, not a shift. D shifts labor supply right. E is a price control that changes the quantity hired along existing curves; it does not shift demand. Fix rule: labor demand shifts only when MRP changes — product price, productivity, or technology; the wage itself only moves you along.
Q5 — A. A wage taker can hire any number of workers at the going market wage, so the labor supply it faces is horizontal at W, and MFC = W. B describes the supply curve facing a monopsonist, not a competitive hirer. C would mean no hiring flexibility at any wage, which contradicts a functioning labor market. D confuses the labor market with the product market. E confuses the firm's labor supply with its labor demand (MRP). Fix rule: competitive hiring → horizontal labor supply at the market wage; upward-sloping supply to the firm signals monopsony.
Q6 — D. Cleaners' MRP = MP × P doubles at every quantity, so labor demand shifts right, raising the equilibrium wage and employment together. A puts the change on the wrong curve — nothing happened to workers' willingness to supply labor. B misses that a product-price change is exactly what shifts the MRP curve. C wrongly imports product-market logic (higher price → less quantity demanded) into the factor market's shift analysis. E is impossible for a pure rightward demand shift along an upward supply curve. Fix rule: product price up → MRP up → factor demand right → wage and employment both rise.
Q7 — A. Compare output per dollar: labor 40/5 = 8, capital 90/15 = 6. Labor delivers more output per dollar, so substitute toward labor. B follows raw marginal products (90 > 40) — the trap this question is built on. C would require the per-dollar ratios to be equal; 8 ≠ 6. D expands output rather than producing the same output more cheaply. E confuses the least-cost rule with the MRP hiring rule — the input mix decision needs only MPs and input prices. Fix rule: least cost = equal MP per dollar; always divide by the input's price before comparing.
Q8 — D. Equal marginal product per dollar spent is exactly the least-cost condition — no reshuffling of inputs can make the current output cheaper. A and B would break the equality and raise cost. C overreaches: least cost says this output is produced cheaply, not that this output level maximizes profit (that requires MRP = MFC for each input). E confuses equalizing marginal products with equalizing marginal products per dollar. Fix rule: MP_L/P_L = MP_K/P_K = least-cost mix; profit maximization is a separate, additional condition.
Q9 — E. Employers hire and retain when MRP ≥ wage, so demonstrating that her work generates more revenue than her pay costs speaks directly to the employer's decision rule. A appeals to seniority, which affects MRP only if experience raises productivity. B cites totals, but hiring logic runs on margins — high totals can coexist with a wage above MRP. C describes her costs, not the firm's benefits; it does not change her MRP. D is an equity comparison that says nothing about whether her MRP covers her wage. Fix rule: the wage case an employer must respect is MRP > W — show revenue added, not tenure, need, or comparisons.
Q10 — B. A monopsonist faces the upward-sloping market supply curve, so each additional hire requires a higher wage paid to the entire staff — total labor cost jumps by more than the new nurse's wage. A describes competition from other employers, the opposite of monopsony. C is true of MRP but explains the demand side, not why MFC exceeds the wage. D confuses product-market monopoly with factor-market monopsony — they are independent. E describes a competitive hirer, where MFC would equal the wage. Fix rule: monopsony MFC > W because the raise for incumbents rides along with every new hire.
Q11 — B. The monopsonist hires where MRP = MFC, which is fewer workers than where MRP crosses supply, and then pays the lower supply-curve wage at that employment level. A and D wrongly assume market power lets the buyer of labor pay more. C cannot happen: restricted hiring and a wage read lower on the supply curve go together. E misses that the wage falls below the competitive level, not just employment. Fix rule: monopsony = fewer workers and a lower wage — quantity from MRP = MFC, wage from the supply curve below.
Q12 — C. The wage is the price on the labor market's own axis; when it changes, the firm slides along its MRP (labor demand) curve, hiring fewer baristas at the higher wage — but the curve itself has not moved. A misapplies product-market shift logic; input prices don't shift the input's own demand curve. B is false: MRP depends on MP and the product's price, not on the wage. D commits the same shift error in the opposite direction. E moves the mistake to the other curve — a wage change doesn't shift labor supply either; it moves along both curves. Fix rule: a factor's own price never shifts that factor's demand or supply — own-price changes are movements along.