A local library sells memberships for the use of its computer lab. Four hundred students purchased a membership when the price was $20. The library estimates that for every $2 increase in the membership fee, 20 fewer students will become members. What should the membership fee be to provide the maxium revenue to the library?

Let x=the number of memberships.

Then, the number decreases by 20 for each 2 dollar increase.

(400-20x)(20+2x)

Differentiate, set to 0 and solve for x