Answer:
Falls:rises.
Step-by-step explanation:
The MU/P (Marginal Utility/Price) ratio for good X is greater than the MU/P (Marginal Utility/Price) ratio for good Y. To achieve consumer equilibrium, the consumer reallocates dollars from the purchase of good Y to the purchase of good X. If the law of diminishing marginal utility holds, the marginal utility of good X falls and the marginal utility of good Y rises.
The law of diminishing marginal utility states that as the unit of a good or service consumed by an individual increases, the additional satisfaction he or she derives from consuming additional units would start decreasing or diminishing as the units of good or service consumed increases.
Also, the marginal utility of goods and services is the additional satisfaction that a consumer derives from consuming or buying an additional unit of a good or service.
Hence, the marginal utility of good X falls and the marginal utility of good Y rises because the consumer no longer derive satisfaction or benefits (utility) from the consumption of good X while he would switch to good Y for satisfaction.