Simone has 20 bills. One fifth of the bills are hundreds, 1/4 of them are fifties, 1/10 of them are $20s. The rest of the bills are tens.

Simone has$780 altogether. How many of each kind of bill does she have

If Simone has 20 bills, then:

1/5 of 20 = 4 [hundreds]

1/4 of 20 = 5 [fifites]

1/10 of 20 = 2 [$20s]

4 * 100 + 5 * 50 + 2 * 20 = 400 + 250 + 40 = 690 [out of $780]

4 + 5 + 2 = 11 [out of 20 bills]

So now we can easily find out she had 9 bills of tens - because 20 - 11 = 9 or because (780 - 690) : 10 = 90 : 10 = 9.