This is a problem that is going to require using a system of equations to solve. A system needs the same amount of equations as variables. To elaborate, if you have 1 variable in one equation then you can solve for it, but if you have 2 variables then you will need 2 equations to solve for both of them.
You have 2 variable because you are "How many of each bill does he have?" That means, how many $1 's and how many $5's.
Call the amount of $1's x and the amount of $5's y.
Since he has 16 bills in all: x + y = 16
Since all the $1's plus all the $5's amount to $44: 1x + 5y = 44
Solve this system for x and y using substitution or elimination.