Answer
$R=(300-25x)(40+5x)$
$\:\:\:\mathrm{or}\:\:\:$
$R=-125x^2+500x+12000$
Work Step by Step
$\mathrm{Revenue\:function:}\:\:\:R=(300)(40)$
If we increase the price of the book by $\:\$5\:,\:25\:$ fewer books will be sold. If we increase the price by "$x\:$ times $\:\$5\:$", then $\:25x\:$ fewer books will be sold.
So, the new price of the book would be $\:40+5x\:$ and the new number of books sold would be $\:300-25x.$
The new revenue function would be:
$R=(300-25x)(40+5x)$
