Question 1157081
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OneFit population = 100 + 20x


ByGym population  = 400 - 10x


100 + 20x = 400 - 10x


10x + 20x = 400 - 100

30x       = 300

  x       =  300/30    = 10.


<U>ANSWER</U>.  After 10 months.
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Solved (correctly)