There are several important information's already given in the question. Based on those given information's the answer to the question can be easily deduced.
Total number of students in the school = 209
Ration of the boys to girls in the school = 6:5
Let us assume the common ratio to be = x
Then
6x + 5x = 209
11x = 209
x = 209/11
= 19
Then
The number of girls in the school = 19 * 5
= 95
So there are a total of 95 girls in the school that has a total of 209 students.