ratio

let the number of boys last year is 6k
then the number of girls last year is 5k
and the number of students last year is 11k

so, the number of boys this year is 6k x 7/6 = 7k
and the number of students last year is 11k x 25/22 = 25k/2

therefore, the increase ratio of the number of girls this year
= (25k/2 - 7k) / (5k)
= (11k/2) / (5k)
= 11/10
= 11 : 10