I am stuck on this question.
Amy purchased four cantaloupes at $2 each and three watermelons at $4 each. If Amy is following the optimal consumption rule, the marginal utility of the fourth cantaloupe and third watermelon are:

How am I supposed to approach this problem?

Google

Something like divide the total utility by what you've got.
We're missing info

The only thing we know about the marginal utility of the fourth cantaloupe and the third watermelon is that they are equal per dollar spent. In other words, if X is the marginal utility of the cantaloupe and Y is the marginal utility of the watermelon, we know that X/2 = Y/4
Without knowing how much utility the other cantaloupes and watermelons generate, you can't determine X and Y. You probably forgot to post some other relevant piece of information.

This post was edited by russian on Jun 29 2016 04:18pm

2 watermelons on the left

I think graphing these are typically good if you don't understand the concept

Other than that marginal utility is the amount you gain (or lose) based on adding one more units or removing some from one of the two things being compared price wise.