If the diameter is 60ft, then the radius is 30ft, so the equation of the circle, if (0,0) is the origin, is

x^2 +y^2 = 30^2

x^2 +y^2 = 900 ------------(i)

dV is a disc of radius x, and height dy.

dV = pi(x^2)*dy

dV = pi(900 -y^2)dy

dy goes from y = -30 to y = -5

So,

V = (pi)INT.(-30 to -5)[900 -y^2]dy

V = 42,542.4 cu.ft. -------------answer.