There are 96 girls.
The ratio of girl to boy is 4:5
If there is 9 in total at a ratio of 4:5 then there will be 4 girls and 5 boys.
So if there is 96 girls;
So initially there were 120 boys.
Now let us say x number of boys arrived.
Total boys = 120+x
Total girls = 96
`G:B = 96:(120+x)`
But it is given that the G:B ratio is 2:3 at the end.
`96:(120+x) = 2:3`
Usually a ratio means the division or fraction of numbers.
`96/(120+x) = 2/3`
`96xx3 = 2(120+x)`
`x = 24`
So 24 boys has arrived.