Finding Number of $1 Bills in Total of $30

John and Maria have a total of $30 in $1 and $5 bills. If the number of $1 bills is 4 less than twice the number of $5 bills, how many $1 bills do they have?

Let $x$ represent the number of $5 bills, and let $y$ represent the number of $1 bills. You can represent this scenario with the following system of equations:

$$y = 2x - 4$$

$$y + 5x = 30$$