GMAT Quantitative Reasoning Practice Test 3

The customer uses the machine for 3 hours and 25 minutes. The first 15 minutes cost him $\mathrm{\$}4$ and he has 3 hours and 10 minutes left, which is $12\times 3=\mathrm{\$}36+10$ minutes. 10 minutes are $1/6$ of an hour, which is $1/6\times 12=\mathrm{\$}2$. The total cost will be: $4+36+2=\mathrm{\$}42$.