ANSWER:
[tex]\begin{gathered} 1x+20y=51 \\ x=6y-1 \end{gathered}[/tex]STEP-BY-STEP EXPLANATION:
We can propose two equations, one regarding the total value of money and the other regarding the total number of bills.
Therefore, it would be:
[tex]\begin{gathered} 1x+20y=51 \\ x=6y-1 \end{gathered}[/tex]