Final answer:
In response to a price increase for peanut brittle, Jill will typically experience a fall in the marginal utility for peanut brittle and a rise in the marginal utility for frozen yogurt due to the substitution effect of consuming these normal goods.
Step-by-step explanation:
When Jill, a careful utility maximizer who consumes peanut brittle and frozen yogurt (both of which are normal goods), experiences a price increase for peanut brittle and adjusts her consumption, we look at two effects: the substitution effect and the income effect. T
he substitution effect will typically cause the marginal utility of the more expensive good (peanut brittle) to fall because Jill will buy less of it in substitution for a cheaper alternative (frozen yogurt), whose marginal utility will rise. However, the income effect of the price rise reduces Jill's real income which also can shift consumption from both normal goods.
If Jill is strictly adhering to the utility-maximizing principle as she adjusts her consumption, her responses would be aligned with the common reactions to price changes in normal goods. That is, the marginal utility of peanut brittle falls because as its price increases, she gets less satisfaction from consuming an additional unit given the higher cost.
Meanwhile, as she substitutes frozen yogurt for expensive peanut brittle, the additional satisfaction (marginal utility) she gains from consuming frozen yogurt rises, assuming no other changes in prices or income.