A right circular cylinder

It should be: To find the time needed for 30 liters of water to evaporate we need to find the surface area of the top of the cylinder: \(2*area\) will be the amount of water that evaporates each hour, thus \(time=\frac{30}{2*area}\).

For example if the surface area of the top is 5m^2, then in one hour 5*2=10 liters of water evaporates hence it'll take 30/10=3 hours 30 liters of water to evaporate.

Complete solution.

A right circular cylinder of 72 cubic meters is completely filled with water. If water evaporates from the cylinder at a constant rate of two liters per hour per one square meter of surface of the top, how long will it take for 30 liters of water to evaporate?

To find the time needed for 30 liters of water to evaporate we need to find the surface area of the top of the cylinder: \(2*area\) will be the amount of water that evaporates each hour, thus \(time=\frac{30}{2*area}\).

On the other hand since \(volume=\pi{r^2}h=72\) then \(area=\pi{r^2}=\frac{72}{h}\). So, basically all we need is ether the area of the surface or the height of the cylinder.

(1) The height of the cylinder is 2 meters. Sufficient.
(2) The radius of the base of the cylinder is \(\frac{6}{\sqrt{\pi}}\) meters --> \(area=\pi{r^2}=36\). Sufficient.

Answer: D.