How do I figure out the total revenue and marginal revenue. Do I just multiply 20+q/200 by q getting 20q+q^2/200? If so, the partial derivative is 20+q/100? Equating that to 20+q/200 gets q=0. What's going on?

First of all, you got the marginal revenue (MR) right.

Then, you let MR = demand. That’s wrong that that is why your results don’t make any sense.

 

A monopoly will produce a quantity so that the marginal revenue = marginal cost. Use that quantity and demand curve (again) to get price .