Answer: She should use 25 millimeters of substance A
This is a case of ratio. Substance A and B are to be mixed in a given ratio such that as A increases, B would similarly increase, not by the same quantity but by the same ratio. That means every time you add 5 parts of A, you need to add 3 parts of B. In effect, if you add 5 parts of A ten times (5 * 10 = 50 parts) you will need to add 3 parts of B ten times also (3 * 10 = 30).
So the
