Start by making a system of equations, with x representing the number of $1 bills and y representing the number of $5 bills.
We know that x + y = 17 because there are 17 total bills, and we know that x + 5y = 57 because the total value is 57.
x + 5y = 57
x + y = 17
Subtract the bottom equation from the top.
4y = 40
y = 10
We now know that he has 10 $5 bills, and we can substitute 10 in either of our original equations to get x.
x + y = 17
x + 10 = 17
x = 7
He has 10 $5 bills and 7 $1 bills.