The number of months when the gyms cost the same is 5 months.
Total cost of joining the gym = registration fee + (cost per month x number of months)
When the gyms costs are the same, the two above equations would be equal to each other:
50 + 20m = 70 + 10m
20m - 10m = 70 - 20
10m = 50
m = 50 / 10
m = 5
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