Answer: 1.6 cheesecakes
Explanation: Opportunity cost is simply the cost of a forgone alternative. It is the cost of an opportunity forgone (and the loss of the benefits that could be received from that opportunity); the most valuable forgone alternative.
If Marv can decorate 8 wedding cakes or 13 cheesecakes, it follows that the opportunity cost of making 8 wedding cakes is 13 cheesecakes. The question asks the cost of making a cake. This is given by:
13/8 = 1.625 cheesecakes
= 1.6 cheesecakes to the nearest tenth as the answer.