Question

Ed has a cylindrical water tank on his farm. The tank is 7 feet deep and has a diameter of 42 feet. If it takes about 7.5 gallons of water to fill 1 cubic foot of space, which is the best estimate of how much water the tank can hold?
A. 9,700 gallons
B. 19,400 gallons
C. 38,800 gallons
D. 72,800 gallons

Answer 1

well, we know the diameter of the cylinder is 42, thus its radius is half that, or 21 feet.  Since we know its radius and height, 7 feet deep, we can get the exact volume of ft³ it takes.

$\bf \textit{volume of a cylinder}\\\\ V=\pi r^2 h~~ \begin{cases} r=radius\\ h=height\\ -----\\ r=21\\ h=7 \end{cases}\implies V=\pi (21^2)(7)$

now, compare that with any of those estimates, keeping in mind that they're just approximations.