To find this, we need to find the volume of the hole.

Volume = base area x depth

Since the hole is circular, the formula for finding its area is:

Area = pi x radius

radius = diameter/2

radius = 15/2

radius = 7.5 inches

Area = pi x radius

Area = 3.14 x 7.5

Area = 3.14 x 56.25

Area = 176.63 square inches

Volume = area x depth

Volume = 176.63 x 42

Volume = 7418.5 cubic inches

Thus, you need 7418.5 cubic inches of concrete to fill the hole.

Volume = base area x depth

Since the hole is circular, the formula for finding its area is:

Area = pi x radius

^{2}radius = diameter/2

radius = 15/2

radius = 7.5 inches

Area = pi x radius

^{2}Area = 3.14 x 7.5

^{2}Area = 3.14 x 56.25

Area = 176.63 square inches

Volume = area x depth

Volume = 176.63 x 42

Volume = 7418.5 cubic inches

Thus, you need 7418.5 cubic inches of concrete to fill the hole.