The volume of the stone is equal to the volume of water that rises in the tub

$V = A_bh = \frac{1}{4}\pi(40^2)(20)$

$V = 8000\pi ~ \text{in.}^3$

$V = 8000\pi ~ \text{in.}^3 \left( \dfrac{1 ~ \text{ft.}}{12 ~ \text{in.}} \right)^3$

$V = 14.544 ~ \text{ft.}^3$ *answer*