A bank teller has 54 $5 and $20 bills in her cash drawer. The value of thebills is $780. How many $5 bills are there?

Let x be the number of $5 bills.

Then the number of $20 bills = 54 - x

5x+ 20(54-x) = 780

Or, 5x +1080 - 20x = 780

Or, -15x = 780 - 1080 = -300

Or, x = -300/-15 = 20

There are 20 $5 bills.