Question: I need a clearer understanding of how to solve: Kelly has 5 quarters and 2 dimes. If she buys a can of pop for 55 cents, how many cents will she have left?
Answer: 
Her quarters are worth 5 * 25 = 125 cents.
The dimes are worth 2 * 10 = 20 cents
Kelly has a total of 125 + 20 = 145 cents
After buying the can of pop, Kelly will have 145 - 55 = 90 cents left.