Answer
2.88 feet
Work Step by Step
We know that the volume of a sphere is:
$\displaystyle V=\frac{4}{3}\pi r^{3}$
We solve for $r$:
$\displaystyle r=\sqrt[3]{\frac{3V}{4\pi}}$
We know that $V=750$ gallons. We convert this to cubic feet:
$V=750\ gallons * 0.1337\ ft^3/gallon=100.275 ft^3$
Now we solve for $r$ in feet:
$r=\sqrt[3]{\frac{3V}{4\pi}}=r=\sqrt[3]{\frac{3*100.275}{4\pi}}=2.88\ feet$
