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