Your original solution is correct.
In the generalization, your set-up is wrong . . .
Look at what we've got . . .How much water must be evaporated from 500 grams of a 10% salt solution
in order to obtain a solution containing 15% salt?
My solution was to make x represent the amount of water to be evaporated.
My equation was: . . Right!
We have grams of solution which is salt.
We evaporate grams of water to get a solution which is salt.
The general equation is: .