Representing a function of a spherical balloon!!?

Question

A spherical balloon with radius r inches has volume defined by the function below. Find a function that represents the amount of air required to inflate the balloon from a radius of r inches to a radius of r + 5 inches. (Give the answer in terms of π and r.)

the originial formula is v= 4/3 pi r^2

Answer 1

Shouldn't it be

V = (4/3)πr³ ?

Required volume of air

= (4/3)π((r+5)³ - r³)

= (4/3)π(r³ + 15r² + 75r + 125 - r³)

= (4/3)π(15r²+75r+125)

= (20/3)π(3r²+15r+25)