A. A recent stock market boom has increased household wealth by $20 billion, which increases consumption by $12 billion, and the marginal propensity to consume in the
economy is equal to 0.6.
In the new equilibrium, aggregate expenditure will increase
by $
billion
b. Rising interest rates reduce domestic consumption by $4 billion and reduce investment by
$7 billion, and the marginal propensity to consume in the economy is equal to 0.7.
In the new equilibrium, aggregate expenditure will decrease
by $
billion.

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answer: marginal utility of the fourth brownie is 0.

explanation: marginal utility is the addtional utility gained from the consumption of one more unit of a good. it is given by,

so, marginal utility at the 4th unit will be given by

since, the values are not given here, you can simply find marginal utility at fourth unit by subtracting total utility at 3rd unit from total utility at 4th unit.