Units and parts

Question:

Class 5A and 5B have the same number of pupils. The ratio of the number of boys in 5A to the number of boys in 5B is 2 : 3. The number of girls in 5A to the number of girls in 5B is 7 : 4. 
(A) Find the ratio of the number of boys to the number of girls in 5A.
(B) If there are 15 fewer girls than boys in 5B, how many pupils are there in 5B?

Solution:

5A boys : 5B boys = 2u : 3u
5A girls : 5B girls = 7p : 4p

5A and 5B same no. of pupils --> 2u + 7p = 3u + 4p --> 1u = 3p
5A boys : 5B boys = 2u : 3u = 6p : 9p

(A) 5A boys : 5A girls = 6 : 7

(B) 5B boys : 5B girls = 9 : 4 
5parts --> 15 
1p --> 3
No. of pupils in 5B is 13p --> 13 x 3 = 39