Economics Today and Tomorrow Advanced
Prep: Master Microeconomic Theory
Practice Questions & Detailed Explanations
Subject: Microeconomic Theory – Consumer Choice, Elasticity, and Market
Structures
Question 1: A consumer has a utility function $U(x, y) = x^{0.5}y^{0.5}$. If the price of good
$x$ doubles and the price of good $y$ remains constant, while the consumer's income also
doubles, what happens to the consumer’s optimal bundle $(x^*, y^*)$?
A) Both $x^*$ and $y^*$ remain unchanged.
B) The consumer buys more of $x$ and less of $y$.
C) The consumer buys less of $x$ and the same amount of $y$.
D) The consumer buys the same amount of $x$ and more of $y$.
Correct Answer: D) The consumer buys the same amount of $x$ and more of $y$.
Explanation: For a Cobb-Douglas utility function $U = x^a y^b$, the demand functions are $x =
\frac{a}{a+b} \cdot \frac{I}{P_x}$ and $y = \frac{b}{a+b} \cdot \frac{I}{P_y}$. With $a=0.5$
and $b=0.5$, $x^ = 0.5 \cdot \frac{I}{P_x}$ and $y^* = 0.5 \cdot \frac{I}{P_y}$. If $I$ doubles
and $P_x$ doubles, the ratio $I/P_x$ remains constant, keeping $x^*$ unchanged. If $I$ doubles
and $P_y$ remains constant, the ratio $I/P_y$ doubles, leading to an increase in $y^*$.*
Question 2: In a perfectly competitive market, a firm faces a total cost function $TC(q) = q^3 -
6q^2 + 15q + 20$. At what minimum price must the market settle for the firm to continue
operating in the short run?
A) $3$
B) $6$
C) $9$
D) $15$
Correct Answer: B) 6
Explanation: A firm continues to operate in the short run if the price is at least equal to the
minimum Average Variable Cost (AVC). The variable cost is $VC = q^3 - 6q^2 + 15q$. Thus,
,$AVC = q^2 - 6q + 15$. To find the minimum, take the derivative: $d(AVC)/dq = 2q - 6 = 0$, so
$q=3$. Plugging $q=3$ into the AVC function: $3^2 - 6(3) + 15 = 9 - 18 + 15 = 6$.
Question 3: Consider a monopolist with a marginal cost $MC = 10$ facing a linear inverse
demand curve $P = 100 - 2Q$. If the government imposes a specific tax of $\$10$ per unit, what
is the effect on the equilibrium price charged to consumers?
A) It increases by $\$10$.
B) It increases by $\$5$.
C) It increases by $\$0$.
D) It decreases by $\$5$.
Correct Answer: B) It increases by $\$5$.
Explanation: The monopolist sets $MR = MC$. $TR = P \cdot Q = (100 - 2Q)Q = 100Q -
2Q^2$, so $MR = 100 - 4Q$. Set $100 - 4Q = 10$, so $4Q = 90$, $Q = 22.5$. $P = 100 -
2(22.5) = 55$. With a tax, $MC$ becomes $10 + 10 = 20$. Set $100 - 4Q = 20$, so $4Q = 80$,
$Q = 20$. The new price $P = 100 - 2(20) = 60$. The price increased from 55 to 60, an
increase of 5.
Question 4: Which of the following conditions must hold for a Nash Equilibrium in a static game
where firms choose quantities (Cournot competition)?
A) Marginal Revenue equals Marginal Cost for each firm, given the other firm's output.
B) The market price is equal to the marginal cost of the least efficient firm.
C) Each firm chooses a quantity that maximizes joint industry profit.
D) Price is equal to the minimum of the long-run average cost curve.
Correct Answer: A) Marginal Revenue equals Marginal Cost for each firm, given the other
firm's output.
Explanation: In a Cournot model, each firm treats the output of the competitor as a fixed
parameter. Therefore, each firm maximizes its own profit by setting its own marginal revenue
equal to its own marginal cost, resulting in a stable quantity combination.
Question 5: A Giffen good is characterized by which of the following?
A) An income effect that reinforces the substitution effect.
B) A positive income elasticity of demand.
,C) An upward-sloping demand curve caused by a strong negative income effect.
D) Being a luxury good that becomes an inferior good at high income levels.
Correct Answer: C) An upward-sloping demand curve caused by a strong negative income
effect.
Explanation: For an inferior good, the substitution effect and income effect work in opposite
directions when the price increases. If the income effect (which encourages more consumption as
"real income" drops) outweighs the substitution effect (which encourages less consumption due
to higher opportunity cost), the total quantity demanded increases as price increases, creating
an upward-sloping demand.
Question 6: In the Edgeworth Box model, the contract curve represents:
A) All points where the indifference curves of both traders are tangent to each other.
B) The set of initial endowments of the two traders.
C) The set of points where trade is impossible.
D) The equilibrium outcome of competitive markets only.
Correct Answer: A) All points where the indifference curves of both traders are tangent to
each other.
Explanation: The contract curve is the locus of all Pareto efficient allocations. At these points,
the marginal rate of substitution (MRS) for both agents is equal, meaning no further mutually
beneficial trades are possible.
Question 7: A firm experiences increasing returns to scale if:
A) Doubling all inputs leads to more than a doubling of output.
B) Increasing one input while holding others constant leads to increased marginal productivity.
C) The marginal product of every input is increasing.
D) Average total cost is constant as output increases.
Correct Answer: A) Doubling all inputs leads to more than a doubling of output.
Explanation: Returns to scale describe what happens to output when all inputs are scaled by the
same factor $\lambda$. If $f(\lambda L, \lambda K) > \lambda f(L, K)$, the production function
exhibits increasing returns to scale.
, Question 8: Under the Bertrand model of oligopoly with homogeneous products, the equilibrium
price is equal to:
A) The monopoly price.
B) The Cournot equilibrium price.
C) Marginal Cost.
D) Average Total Cost.
Correct Answer: C) Marginal Cost.
Explanation: In Bertrand competition with identical goods, firms compete on price. Even with
two firms, the incentive to undercut the rival leads to a price war until price reaches marginal
cost, at which point no firm has an incentive to deviate.
Question 9: If a negative externality exists in a market, the competitive market equilibrium is
inefficient because:
A) Marginal Social Cost exceeds Marginal Private Cost.
B) Marginal Social Benefit is less than Marginal Private Benefit.
C) The price is too low to cover the producer's fixed costs.
D) The government has not imposed a quota on production.
Correct Answer: A) Marginal Social Cost exceeds Marginal Private Cost.
Explanation: When a negative externality (like pollution) exists, the cost to society of producing
an extra unit includes the private cost to the producer plus the external damage to others.
Market equilibrium occurs where $P = MPC$, which is less than $MSC$, resulting in
overproduction relative to the social optimum.
Question 10: According to the Coase Theorem, if transaction costs are zero, an efficient outcome
will be achieved regardless of the initial allocation of property rights if:
A) The government enforces strict price ceilings.
B) Affected parties can bargain costlessly.
C) Both parties have identical utility functions.
D) The externality is internal to the firm's own production process.
Prep: Master Microeconomic Theory
Practice Questions & Detailed Explanations
Subject: Microeconomic Theory – Consumer Choice, Elasticity, and Market
Structures
Question 1: A consumer has a utility function $U(x, y) = x^{0.5}y^{0.5}$. If the price of good
$x$ doubles and the price of good $y$ remains constant, while the consumer's income also
doubles, what happens to the consumer’s optimal bundle $(x^*, y^*)$?
A) Both $x^*$ and $y^*$ remain unchanged.
B) The consumer buys more of $x$ and less of $y$.
C) The consumer buys less of $x$ and the same amount of $y$.
D) The consumer buys the same amount of $x$ and more of $y$.
Correct Answer: D) The consumer buys the same amount of $x$ and more of $y$.
Explanation: For a Cobb-Douglas utility function $U = x^a y^b$, the demand functions are $x =
\frac{a}{a+b} \cdot \frac{I}{P_x}$ and $y = \frac{b}{a+b} \cdot \frac{I}{P_y}$. With $a=0.5$
and $b=0.5$, $x^ = 0.5 \cdot \frac{I}{P_x}$ and $y^* = 0.5 \cdot \frac{I}{P_y}$. If $I$ doubles
and $P_x$ doubles, the ratio $I/P_x$ remains constant, keeping $x^*$ unchanged. If $I$ doubles
and $P_y$ remains constant, the ratio $I/P_y$ doubles, leading to an increase in $y^*$.*
Question 2: In a perfectly competitive market, a firm faces a total cost function $TC(q) = q^3 -
6q^2 + 15q + 20$. At what minimum price must the market settle for the firm to continue
operating in the short run?
A) $3$
B) $6$
C) $9$
D) $15$
Correct Answer: B) 6
Explanation: A firm continues to operate in the short run if the price is at least equal to the
minimum Average Variable Cost (AVC). The variable cost is $VC = q^3 - 6q^2 + 15q$. Thus,
,$AVC = q^2 - 6q + 15$. To find the minimum, take the derivative: $d(AVC)/dq = 2q - 6 = 0$, so
$q=3$. Plugging $q=3$ into the AVC function: $3^2 - 6(3) + 15 = 9 - 18 + 15 = 6$.
Question 3: Consider a monopolist with a marginal cost $MC = 10$ facing a linear inverse
demand curve $P = 100 - 2Q$. If the government imposes a specific tax of $\$10$ per unit, what
is the effect on the equilibrium price charged to consumers?
A) It increases by $\$10$.
B) It increases by $\$5$.
C) It increases by $\$0$.
D) It decreases by $\$5$.
Correct Answer: B) It increases by $\$5$.
Explanation: The monopolist sets $MR = MC$. $TR = P \cdot Q = (100 - 2Q)Q = 100Q -
2Q^2$, so $MR = 100 - 4Q$. Set $100 - 4Q = 10$, so $4Q = 90$, $Q = 22.5$. $P = 100 -
2(22.5) = 55$. With a tax, $MC$ becomes $10 + 10 = 20$. Set $100 - 4Q = 20$, so $4Q = 80$,
$Q = 20$. The new price $P = 100 - 2(20) = 60$. The price increased from 55 to 60, an
increase of 5.
Question 4: Which of the following conditions must hold for a Nash Equilibrium in a static game
where firms choose quantities (Cournot competition)?
A) Marginal Revenue equals Marginal Cost for each firm, given the other firm's output.
B) The market price is equal to the marginal cost of the least efficient firm.
C) Each firm chooses a quantity that maximizes joint industry profit.
D) Price is equal to the minimum of the long-run average cost curve.
Correct Answer: A) Marginal Revenue equals Marginal Cost for each firm, given the other
firm's output.
Explanation: In a Cournot model, each firm treats the output of the competitor as a fixed
parameter. Therefore, each firm maximizes its own profit by setting its own marginal revenue
equal to its own marginal cost, resulting in a stable quantity combination.
Question 5: A Giffen good is characterized by which of the following?
A) An income effect that reinforces the substitution effect.
B) A positive income elasticity of demand.
,C) An upward-sloping demand curve caused by a strong negative income effect.
D) Being a luxury good that becomes an inferior good at high income levels.
Correct Answer: C) An upward-sloping demand curve caused by a strong negative income
effect.
Explanation: For an inferior good, the substitution effect and income effect work in opposite
directions when the price increases. If the income effect (which encourages more consumption as
"real income" drops) outweighs the substitution effect (which encourages less consumption due
to higher opportunity cost), the total quantity demanded increases as price increases, creating
an upward-sloping demand.
Question 6: In the Edgeworth Box model, the contract curve represents:
A) All points where the indifference curves of both traders are tangent to each other.
B) The set of initial endowments of the two traders.
C) The set of points where trade is impossible.
D) The equilibrium outcome of competitive markets only.
Correct Answer: A) All points where the indifference curves of both traders are tangent to
each other.
Explanation: The contract curve is the locus of all Pareto efficient allocations. At these points,
the marginal rate of substitution (MRS) for both agents is equal, meaning no further mutually
beneficial trades are possible.
Question 7: A firm experiences increasing returns to scale if:
A) Doubling all inputs leads to more than a doubling of output.
B) Increasing one input while holding others constant leads to increased marginal productivity.
C) The marginal product of every input is increasing.
D) Average total cost is constant as output increases.
Correct Answer: A) Doubling all inputs leads to more than a doubling of output.
Explanation: Returns to scale describe what happens to output when all inputs are scaled by the
same factor $\lambda$. If $f(\lambda L, \lambda K) > \lambda f(L, K)$, the production function
exhibits increasing returns to scale.
, Question 8: Under the Bertrand model of oligopoly with homogeneous products, the equilibrium
price is equal to:
A) The monopoly price.
B) The Cournot equilibrium price.
C) Marginal Cost.
D) Average Total Cost.
Correct Answer: C) Marginal Cost.
Explanation: In Bertrand competition with identical goods, firms compete on price. Even with
two firms, the incentive to undercut the rival leads to a price war until price reaches marginal
cost, at which point no firm has an incentive to deviate.
Question 9: If a negative externality exists in a market, the competitive market equilibrium is
inefficient because:
A) Marginal Social Cost exceeds Marginal Private Cost.
B) Marginal Social Benefit is less than Marginal Private Benefit.
C) The price is too low to cover the producer's fixed costs.
D) The government has not imposed a quota on production.
Correct Answer: A) Marginal Social Cost exceeds Marginal Private Cost.
Explanation: When a negative externality (like pollution) exists, the cost to society of producing
an extra unit includes the private cost to the producer plus the external damage to others.
Market equilibrium occurs where $P = MPC$, which is less than $MSC$, resulting in
overproduction relative to the social optimum.
Question 10: According to the Coase Theorem, if transaction costs are zero, an efficient outcome
will be achieved regardless of the initial allocation of property rights if:
A) The government enforces strict price ceilings.
B) Affected parties can bargain costlessly.
C) Both parties have identical utility functions.
D) The externality is internal to the firm's own production process.
