Call the current situation 'before' and the supposition/'if' case 'after'
Before: Girls 60%, Boys 40%
60% = 60/100 = 3/5, 40% = 40/100 = 2/5
So ratio of Girls to Boys = 3 : 2
After : Girls 70%, Boys 30%
70% = 70/100 = 7/10, 30% = 30/100 = 3/10
So ratio of Girls to Boys = 7 : 3
Since boys remain unchanged, make their units the same. Common multiple of 2 and 3 → 6
Before : Girls to Boys = 3 : 2 = 9 : 6
After : Girls to Boys = 7 : 3 = 14 : 6
Difference between 'before' and 'after' for girls = 14 units - 9 units = 5 units
5 units = 10 people
1 unit = 10 people ÷ 5 = 2 people
Number of children in the classroom = 9 units + 6 units = 15 units = 15 × 2 = 30