Question

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?

Answer 1

There are sixteen $20 bills.

Explanation:

Denote number of $10 bills as x and number of $20 bills as y.

The situation becomes

`10x + 20y = 680`

with `x+y = 52`

We now have a pair of simultaneous equations which is easy to solve. We multiply the second by 10, yielding:

`10x+10y = 520`

and subtract it from the first, leaving:

`10y = 160`

`therefore y = 16`

substitution into either equation then produces that `x=36`