6/12 or 4/8 is a possible answer
8 s -11 s + 6= - 3 s + 6=
= 3( - s +2)
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

, we find that x=25 and y=50. The monthly fee is 50 dollars.