Question

A cylindrical water tank is 12 feet high and 10 feet in diameter. the entire tank, inside and out, is to be painted. if a gallon of paint covers 200 sq ft, how many gallons will be needed?

Answer 1

if we paint the tank on the outside, we'll be covering SA of surface area, however we're also painting it on the inside, so the area inside assuming the material thickness is negligible, we'll be covering SA as well, so we'll be needing SA + SA = 2SA ft² of paint to do the whole thing.

so, its diameter is 10, that means its radius is 5.

$\textit{surface area of a cylinder}\\\\ SA=2\pi r(h+r)~~ \begin{cases} r=radius\\ h=height\\[-0.5em] \hrulefill\\ h=12\\ r=5 \end{cases}\implies \begin{array}{llll} SA=2\pi (5)(12+5)\implies SA=170\pi \\\\\\ \stackrel{\textit{twice as much}}{2SA=340\pi } \end{array}$

now, hmmm we know that 1 gallon can do 200 ft², how many gallons are there in 340π ft²?

$\begin{array}{ccll} gallons&ft^2\\ \cline{1-2} 1 & 200\\ x& 340\pi \end{array} \implies \cfrac{1}{x}~~=~~\cfrac{200}{340\pi } \\\\\\ 340\pi =200x\implies \cfrac{340\pi }{200}=x\implies \stackrel{gallons}{5.34}\approx x$