How many grams of water must be evaporated from 100 kilograms of a 40% salt solution to produce a 50% salt solution?
Mathematically, the answer is 20 000 g of water.
Practically speaking, this is impossible, because the maximum solubility of salt in water is 26 % by mass.
But let's do the mathematical exercise.
We will also assume that the density of the solution remains constant (even though the density changes with concentration!).
A 40 % concentration means
You want to remove
You would have to evaporate 20 kg of water.