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Katie loves swimming. Every afternoon, she visits the local swimming pool for a swim. The entry cost to the pool costs Katie $10 but she can swim for as long as she wants. Once she has entered the pool and paid the fee, Katie wonders how many hours should she spend swimming if she were to think like an economist. She expects to gain an incremental benefit of $21 from the first hour of swimming, then gains subsequent incremental benefits of $17 from the second, $11 from the third and $4 from the fourth. For every hour spent swimming, it will cost Katie $6 as she could have spent the time doing her homework.

In determining how many hours Katie should be swimming, the $10 entry fee to the pool is a/an . Typee A for Average cost, M for Marginal cost, S for Sunk cost or O for Opportunity cost.

Using marginal analysis, what is the optimal amount of hours Katie should spend swimming? .

The maximum surplus for Katie, from spending the number of hours you found in part b, is $. Answer to the nearest whole number (with no decimal places)