We need to construct the system of equations in order to solve that problem. Let's denote one-time membership fee with x and monthly fee with y. Then we can write that John paid x+6y=325 and Abigail paid x+9y=475. Solving the system of equations
![\left \{ {{x+6y=325} \atop {x+9y=475}} \right.](https://img.qammunity.org/2019/formulas/mathematics/high-school/ddpcob765clmgur59a6cp9fh1z4mcertp7.png)
, we find that x=25 and y=50. The monthly fee is 50 dollars.