GMAT Quantitative Reasoning Practice Test 1

First calculate the number of letters completed by 30 typists in 20 minutes. Let $\mathit{x}=$ the number of letters typed by 30 typists and set up the proportion typists/letters $=\frac{20}{48}=\frac{30}{\mathit{x}}$. Cross-multiply to get $20\mathit{x}=1,440$. Divide both sides by 20 and get $\mathit{x}=72$. Since 20 minutes is one-third of an hour, multiply $72\times 3=216$ to get the total letters for one hour.