Question 888167
If you have 40 and you add 5, you now have 45 students.  4:5 is 9.  45/9 = 5.  Your new ratio is 4:5 or 20 girls:25 boys.
Your previous ratio was 2:3 (5 total).  40/5 = 8
Previously there were 8*2 girls: 8*3 boys, or 16:24.  
To get from the old ratio to the new one you added 4 girls and one boy.