Suppose: there used to be x students in the school.

There are former female students (7/12) x.

Later, the number of female students was (7/ 12)x+15, and the total number was x+15.

Countable equation: (7/12) x+15/(x+15) = 3/5.

x=360

That is, the training school originally had 360 students.

Analysis: Later girls: Later total =3:5.