They’re all adding 5, so 18+5=23
So I think 23
Your answer is:

This is because there are 40 boxes in total. The chance of a white box being handed out is 10/40. If a white box is handed out as a prize, there are going to be 39 boxes left, but only 9 of those 39 boxes would be white.
Ideally, tree diagrams should be used to solve such problems.
390, because 10% of 1300 is 130 so 130 times 3 = 390