#### Answer

50 dollars per phone

#### Work Step by Step

Allow x to be the number of additional phones that are sold.
R = (price per phone)(number of phones sold)
number of phones sold = 500 +20x
price per phone = 75-x
Multiply (500+ 20x)(75-x) together to get:
R = 37500 +1000x -20x^2
Then, using the following equation, use the axis of symmetry formula to find the x coordinate that maximizes the revenue:
x = -b/2a simplifies to:
x= -1000/2(-20) simplifies to:
x= 25
Once you have the value of x, plug it back into the equation 75 - x, which represents the price per phone.
75-25 =50
Therefore, the price should be 50 dollars per phone.