There is a ratio of number of boys to girls 3:5. After adding 5 boys and 3 girls, the ratio become 5:7. How many girls were there at first?
b/g = 3/5
(b+5)/(g+3) = 5/7
b = 3g/5
(3g/5 + 5)/(g+3) = 5/7
(3g+25)/(g+3) = 25/7
7(3g+25) = 25(g+3)
21g + 175 = 25g + 75
4g = 100
g = 25
so, b=15
15/25 = 3/5
20/28 = 5/7