There are 36 boys.
Since the ratio is 6:5 and the question wants to know how many boys are in a class if there are 30 girls then all you have to do is divide 30 by 5 to get 6 (which is a key factor in this equation). After that you multiply the 6 by the original number of boys (which is also six) to get a total of 36 boys. This is an equivalent expression to the original ratio.
Original:
6:5
When there are 30 Girls:
36:6
When 36:5 is divided by 6 you get 6:5 which is the original ratio.
Hope this helps and have a nice day.
-R3TR0 Z3R0
