A cash register contains $5 bills and $20 bills with a total value of $180 . If there are 15 bills total, then how many of each does the register contain?

Let f be the number of $5 dollar bills and t be the number of $20 bills. We're given the following equations:

f + t = 15

5f + 20t = 180

We can solve this system of equations 3 ways. We get t = 7 and f = 8.