How to express "x out of y"?
If you want to express a percentage:

（某集体之中）有十分之七的人去了健身房。
或者：（平均下来）十个人里面有七个去了健身房。
70% of them went to the gym.

If you want to express a number:

这十个人里面有七个去了健身房。
(There are exactly 10 people in total, and exactly 7 people went to the gym.)