28 students....ratio of boys to girls is 2 to 5...added = 7
2/7(28) = 56/7 = 8 boys
5/7(28) = 140/7 = 20 girls
so right now, there is 8 boys and 20 girls (8/20 = 2/5)
so lets add 2 boys...making it 30 students
10 boys and 20 girls = 10/20 = 1/2 or 1 to 2
answer is : u have to add 2 boys to get a ratio of boys to girls = to 1:2