39 Primary 1 pupils and 35 of Primary 2 pupils went on a school excursion. The number of Primary 2 boys was 3 times the number of Primary 1 boys, and there were 20 more Primary 1 girls than Primary 2 girls. How many less Primary 2 girls than Primary 2 boys went on the school excursion?

Primary 1		Primary 2
39		35
Boys	Girls	Boys	Girls
1 u	1 p + 20	3  u	1 p

Total number of pupils in Primary 1 = 1 u + 1 p + 20

1 u + 1 p + 20 = 39
1 u + 1 p = 39 - 20
1 u + 1 p = 19 --- (1)

Total number of pupils in Primary 2 = 3 u + 1 p

3 u + 1 p = 35 --- (2)

(2) - (1)
(3 u + 1 p) - (1 u + 1 p) = 35 - 19
3 u - 1 u = 16
2 u = 16

1 u = 16 ÷ 2 = 8

Number of boys in Primary 2
= 3 u
= 3 x 8
= 24

Number of girls in Primary 2
= 1 p
= 35 - 3 u
= 35 - 24
= 11

Number of less girls than boys in Primary 2
= 24 - 11
= 13

Answer(s): 13