I think it is 25% but you need to work out ;)
Well..

get both, check their difference, that's how much more tax you'd be paying from 7.5% to 8% on $10
X=4 so your problem now would be 3+4=7
Hope this helps!
So in short your answer is 4
What i do know is it that its not c, or d its to high and it is a or b.