Some $10 bills and some $20 bills are in a shoe box for a total of 52 bills. The total amount is $680. How many bills are $20?
There are sixteen $20 bills.
Denote number of $10 bills as x and number of $20 bills as y.
The situation becomes
We now have a pair of simultaneous equations which is easy to solve. We multiply the second by 10, yielding:
and subtract it from the first, leaving:
substitution into either equation then produces that