Hello,
Let denote
h the number of men in the concert hall (before leaving)
f the number of women in the concert hall (before leaving)
1/5 h quit =60/2
15/100 f =60/2
h=30*5=150
f=30*100/15=200
Tey were 150+200 = 350 humans in the hall.
(why h : homme, f:femme you may call m and w)
