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 [tex] \left \{ {{x+6y=325} \atop {x+9y=475}} \right. [/tex], we find that x=25 and y=50. The monthly fee is 50 dollars.
