GMAT Quantitative Reasoning Practice Test 3

Define $\mathit{X}$ as the number of dimes that John used. Just a reminder, dimes are 10 cents coins. The number of quarters that he used is $\left(55-\mathit{X}\right)$. We can write the following equation: $10\mathit{X}+25\left(55-\mathit{X}\right)=1000$. Notice that 1000 is the money he spent in cents. Therefore $-15\mathit{X}=-375,\mathit{X}=25$.
The number of dimes is 25 and the number of quarters is $55-25=30$. The difference between the amounts is 5 coins.