How many kilograms of water should be evaporated from 84kg of a 5% salt solution to make it a solution that is 35% salt?

 For 84 Kgs  salt is 5% =4.2 Kgs.

So water is  84-4.2=79.8 Kgs.

35% salt in Hgs =0.35*84=29.4 Kgs.

So water content =84-29.4=54.6 Kgs.

So water to ve evoporated =79.8-54.6

=25.2 Kgs.