This was a question on a microeconomics quiz.

This was a question on a microeconomics quiz. Can someone help me with the formula on how to reach this answer. Any help would be great.

Amy purchased 4 cantaloupes at $2 per cantaloupe and 3 watermelons at $4 per watermelon. If Amy is following the optimal consumption rule, the marginal utility of the fourth cantaloupe and third watermelon are:

The correct answer was: 12 and 24, respectively