Your piggy bank has a total of 40 coins in it; some are dimes and some are nickels. If you have a total of $3.25, how many dimes do you have?

For every dollar there are 10 dimes. And every 20 nickels make a dollar as well.  10 dimes + 20 nickels are 30 coins, adding up to two dollars.  25 dimes ($2.50) + 15 nickels ($0.75) would make a total of 40 coins adding up to $3.25  Therefore there is a total of 25 dimes.

You can often work this sort of problem in your head. If all 40 coins were nickels, the amount would be $2.00. You have $1.25 more than that. Trading one of the nickels for a dime adds $.05 to the value without changing the number of coins. To add $1.25 to the value, you must trade for $1.25/$.05 = 25 dimes.