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?

CornyLou,

1.  I believe you and Rock are correct in parts A and B. To get part C, you partially differentiate the answer to B, and set it equal to 0.  You should get 0 = 1/100 - 100/Q^2; therefore Q = 100.

2. Regarding getting different prices under the Total and Marginal cost curves.   I think the Marginal Cost Curve would more accurately be described as "short term marginal cost curve," and the Average Total Cost Curve could be called the "long term marginal cost curve."  Since we've taken into accout the 100 in Fixed Costs, and in the long run all costs are variable.  For part D we want to use the Average Total Cost Curve, as we want to take these fixed costs into account.