Answer:
x < 3/2 month
Step-by-step explanation:
Let
x = number of months
Planet fitness:
25 + 10x
Retro fitness:
10 + 20x
Equate
25 + 10x < 10 + 20x
25 - 10 < 20x - 10x
15 < 10x
x < 15/10
x < 3/2 month
It is less expensive to join planet when the number of months is less than 3/2 months