Question 655454
The 5 in "3:5" means 5 parts.  Boys to girls = boys:girls.  Then the 3 represents boys and the 5 is girls.

They tell you there are 275 girls.  So, the 5 represents 275 girls.  If 5 parts = 275 then 1 part = 275/5 = 55 girls.  1 unit = 55. 

There are 3:5 boys.  So there are 3x55 boys or 165 boys.