How many ounces of water should be evaporated from 90 ounces of a 10% salt solution in order to make a 15% salt solution?

30 ounces of water

Explanation:

Applying the equation for the calculation of dilutions C1 * V1 = C2 * V2, we can know the final volume of our solution to obtain a concentration of 15%, knowning the final volume we can subtract this volume from the initial one in order to know how much water we should evaporate from our system.

V2 = (0.10*90 ounces) / 0.15 = 60 ounces final volume

Volume of water needed to evaporate = 90 ounces - 60 ounces = 30 ounces of water