120+40x=70x
Subtract 40 from 70 and you get 30
Then you divide 30 into 120 and you get 4.
So in 4 months he will pay the same.
Once you get that.
You plug 4 into. 120+40(4)=280
As well as the other gym option. Which is the 70 per month. So 70(4)=280.
So in the end he will end up paying the same amount for both gyms. So either one he chooses will be a win win. Even with the 120 joining fee. I do hope this help.