Question

The price p (in dollars) and the quantity x sold of a certain product obey the demand equation:

Answer 1

X = -20p + 500
20p = 500 - x
p = (500 - x)/20

R = xp = x(500 - x)/20 = (500x - x^2)/20

Answer 2

X = - 20 p + 500
First we have to express p as a function of x:
20 p = - x + 500  / : 20
p = - x/20 + 25
The revenue:
R = x p
R = x ( - x/20 + 25 ) = - x²/20 + 25 x