The volume of a sphere varies directly with the cube of its radius. A balloon of radius 5 centimeters has volume $\dfrac{500\pi}{3}$ cubic centimeters, or about 524 cubic centimeters. Find a formula for the volume, $V\text{,}$ of a sphere as a function of its radius, $r\text{.}$ Use “pi” for $\pi\text{.}$