How much water must be evaporated from 24 ounces of a 5% salt solution to make a 6% salt​ solution?

In order for the function to be linear, what must m be and why? m = 22 because the rate of change is 1. m = 20 because the rate of change is 3. m = 22 because the rate of change is -1. m = 20 because the rate of change is -3.