35.540 cubic inches
First we find the volume of the cylindrical container:
V = π(r^2)h
d = 3
r = 3/2 = 1.5
V = π(1.5^2)(8.5)
= 60.083 cubic inches
Then we find the volume of three tennis balls:
volume of one ball (sphere):
V = (4/3)π(r^3)
d = 2.5
r = 2.5/2 = 1.25
V = (4/3)π(1.25^3) = 8.181
volume of three spheres:
3 × V = 3 × 8.181
= 24.543 cubic inches
Then we get the difference to find the unused volume of the container:
60.083 - 24.543 = 35.540 cubic inches