Hello, earachefl!

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: .