A manufacturer can produce digital recorders at a cost of 60 dollars each. It is estimated that if the recorders are sold for P dollars a piece, consumers will buy q= 150-p recorders each month.

Question

What is the average rate of profit obtained as the level of production increases from q=0 to q=15?

I first expressed the manufacturer's profit as a function of q with the formula: P=(150-q)q-60. I thought the next step was to take the derivative and then plug in 0 and 15. That isn't working. Any ideas?