The solution is 15%.
Here is the explanation :
To find the minimum, you have to think in the other way.
30% are not wearing a blue pullover, 25% are not wearing blue pants, 15% are not wearing a blue hat and 15% are not wearing a blue coat.
30 + 25 + 15 + 15 = 85
Maximum 85% of the girls are not entirely wearing blue. So there is a minimum of 15% of the girls wearing just blue.
