A manufacturer can produce tape recorders at a cost of $20 apiece. It is estimated that if the tape recorders are sold for p dollars apiece, consumers will buy q=120-p recorders each month.
a) Express the manufacturer's profit P as a function of q. The book gives the answer: (P=(120-q)q-20q )
b) At what rate is profit changing when q=20 recorders are produced ?Is the profit increasing or decreasing at this level of production ? Answer:($60 per unit,increasing)
I need the solution pls



I'm not understang in English if along the words.