Single supplement bug

Hello, I have a problem with single supplement (for example,if you put the cost of 1 adult 100$ child 100$ and the over payment for single supplement+50% ) it should be when 1 adult and 1 child must be 250$ but it turns out 300$. I think the problem is that it adds to 50% and to adults and to children, although it should not be so.

Thanks in advance
Iosif from Russia))