In order to write down the cost of joining to each Gym, you take into account both cost of sign up and cost per month. If you chose x as the variable that determines the number of months, you can write algebraically the costs for each gym as follow:
silver's gym costs = 65 + 50x
solar fitness costs = 20 + 55x
That is, the cost is cost of sign up plus cost of merbership per month (variable x)
