Boys : Girls = 1 : 2
2x - (1/2)x = 36
(3/2)x = 36
x = 24
Boys : Girls = 24 : 48
Since half of the boys dropped there are 12 boys and 48 girls.
Since 6 of the girls dropped, there are 12 boys and 42 girls at the end of the semester.
The ratio of girls to boys is 42 : 12 = 7 : 2