this is a weird question. by simply looking at the it, the answer would be 300-200=100.
I don't think there would be such a question. Instead, it might be: what volume of 5% Dextrose soln should be mixed with 20% dextrose to get 300ml of 15% dextrose soln. In this case, you can solve it like this:
300ml*15%=Xml*5%+(300-X)ml*20%, where X is volume of 5% of dextrose. The idea is pure dextrose should be same on both sides of the equation. If you use Xml of 5%, then 20% soln would be (300-X)ml.
