Gina has $72 in $1 bills, $5 bills, and $20 bills. If she has three times as many $1 bills as she has $5 bills, and only half as many $20 bills as $5 bills, how many of each type of bill does each have?

Total number of bills Gina has = $72

Let the number of $5 bills = X

Number of $1 bills = 3X

Number of $20 bills = X/2

Then

X+3X+X/2=72

4X+X/2=72

8X/2+X/2=72

9X/2=72

X=72*2/9

X=16

So

The number of $5 bills = 16

The number of $1 bills = 48

The number of $20 bills = 8