Explanation:
The ratio is 4:3
First you would do 4 + 3 which is 7
Then you would do 35 ÷ 7 which is 5
Finally we would multiply 5 to the ratio 4:3
So 7 × 4 is 28
And 7 × 3 is 21
So the new ratio is 28:21
So there are 21 girls in the class
Hope this makes sense