Tim spends his income on donuts (D) and coffee (C). Coffee is $2 per cup and donuts are $1 each. Assume that Tim has $10 to spend, and his utility function is given by (D, C)=D0.5C0.5 . For this utility function, D=0.5D−0.5C0.5 and C=0.5D0.5C−0.5 . a. Calculate the optimal number of donuts and coffee for Tim to purchase. Assume that he can purchase partial donuts or cups of coffee. Round answers to two places after the decimal where necessary.

Optimal number of donuts = 5 Donuts

Optimal cups of coffee = 2.5 cups.

Explanation:

Optimal numbers of donuts and coffee can be calculated as follow

First, we need to determine the budget constraint as below

M = ( P(D) x D ) + ( P(C) x C )

Placig values in the formula

10 = D + 2C

Now make utility function as:

U(D,C) = D0.5 C0.5

Marginal Utility donuts

MU(D) = 0.5D-0.5C0.5

Marginal Utility Coffee

MU(C) = 0.5D0.5C-0.5

The formula for marginal rate of substitution

(MRSD,C)= MU(D) / MU(C)  = 0.5D - 0.5C0.5 / 0.5D0.5C - 0.5  = C/D

Now calculate the optimal consumption level  

MRSD,C = P(D) / P(C)

C/D = 1/2

D = 2C (Equation 1 )

Placing the value of D resulted from equation 1, in the budget constraint we as below

10 = D + 2C

10 = 2C + 2C

10 = 4C

C = 10/4 = 2.5

NOw place the value of C in equation 1

D = 2C = 2(2.5) = 5

Optimal number of donuts = 5 Donuts

Optimal cups of coffee = 2.5 cups.

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