A manufacturer produces bolts of fabric with a fixed width. The quantity q of this fabric (measured in yards) that is sold is a function of the selling price p (in dollars per yard), so we can write
q = f(p).
Then the total revenue earned with selling price p is
R(p) = pf(p).
Given
f(15) = 8000
and
f '(15) = −330,
find
R'(15).

R'(15) =