Math
posted by Jon .
These review questions are stumping me.
How much water must be added to 50 ML of a 35% acid solution to produce a mixture that is 20% acid.
Could it be as simple as 15% water?

how much acid initially?
.35*50ml= 17.5ml
How much water?
.65*60=32.5ml
.20(32.5+X)=17.5
solve for X 
I got the answer to be 55 ml, but can you please explain how you knew to do that?