Question 1079791
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The question in this post is formulated incorrectly and therefore makes no sense, until the term "completely inflate the ball" is determined.


The amount of air (the mass of air that must be pumped into the ball) depends on pressure inside the ball.



The answer by the other tutor is wrong and demonstrate that the person doesn't know Physics and even does not possess 
correct physical intuition in the area.


The relation between the mass of the gas, the pressure and the volume is given by the gas law PV = nRT
(see the link https://en.wikibooks.org/wiki/General_Chemistry/Gas_Laws#Ideal_Gas_Law).


Probably, the correct question would be "what is the volume of the ball having the radius of 4 inches ?",

and the answer is V = {{{(4/3)pir^3}}}.