In order to solve this problem, I believe it's best to
Create a proportional relationship
Let x be the amount of people in the gym.
40/100= 12/x
(40/100 Is equal to 40 %)
Now cross multiply.
12×100= 1200
40×x= 40x
Now Divide
1200÷40=30
So the answer is
30 people.
Hope this helps
~JZ