solve for paint mixture?

Question

A certain paint mixture of mass 135 kg contains 20% solids suspended in water. How many kilograms of water must be allowed to evaporate in order to raise the concentration of solids to 25%?

Answer 1

20% of 135kg is 27kg.
This means there are 27kg of solids in the paint.
If the concentration of solids is to be 25%, it means we have 75% of water.
Now doing this as a simple ratio problem...if 1/4 is 27kg, then to get 3/4, multiply by 3.
27*3 = 81kg.
That means there is to be 81kg of water LEFT.

Amount of water evapourated = 135 - 81
= 54kg