GMAT Quantitative Reasoning Practice Test 3

Rick came to the bank with 70 bills and left with 10 and therefore he deposited 60 bills.
Define $\mathit{X}$ as the number of $\mathrm{\$}5$ dollar bills that he deposited and so $\left(60-\mathit{X}\right)$ is the number of $\mathrm{\$}15$ bills that he deposited.
We can write the following equation: $5\mathit{X}+15\left(60-\mathit{X}\right)=850$,

The number of $\mathrm{\$}15$ dollar bills is $60-5=55$.