The answer is 6.
If we would write it in a fraction form then convert is to its simplest term (ratio is another way of writing a fraction), the proportion will be 5/2.
In the missing proportion's case, the numerator is 15, so we should find a denominator such as when you convert them to lowest term, the answer would be the same than that of the original. So 15/6 = 5/2.
That is why the answer is 6.
NOTE: Logic can only be used when the numbers given are lesser, there is always a formula.
But I assure you, this is right! :)