let

w = grams of water that must evaporate

100 kg = 100*10^3 g = 10^5 g

the new solution will have 10^5 - w grams and will be 50% salt

and since the water is 0% salt solution we can write

0.40*10^5 - 0.0*w = 0.50(10^5 - w)

by simplifying and solving the equation we find

w = 20,000 g

click here to see the step by step solution of the equation

20,000 g of water must be evaporated.