You have 10 cups of a cleaning solution that is 10% white vinegar and 90% water. you want the mixture to be 50% white vinegar and 50% water. how many cups of white vinegar do you need to add to the mixture? cups

Whatever% of anything is just (whatever/100) * anything.

let's say we'll add "x" cups of pure vinegar, pure vinegar meaning is 100% vinegar.

how much vinegar is there in "x" cups of 100% vinegar?  well (100/100) * x, or just "x".

how much vinegar is there in 10 cups of 10% vinegar?  well, (10/100) * 10, or just 1.

say after adding the "x" cups to the 10 cups, we end up with "y" cups, of 50% of vinegar.

how much is 50% of y?  well, (50/100) * y or 0.5y.

bearing in mind that x+10 = y, both cups amounts added up, and that x+1=0.5y, both vinegar amounts added up, thus 

25. 22 + 1 2 25