In this case we know that V = 75(pi) mm^3 and h = 9m = 9000mm (to put everything in mm), so we can write:
Now we have to solve this equation for r:
Since we have pi on both sides of the equation, we can eliminate it:
(75)(3) = 9000 r^2
r^2 = 225/9000 = 0.025
r = sqrt(0.025) = 0.1581
r = 0.1581
Answer:
r = 0.1581 mm