sorry sir what is your question??

You substituted values of x and y in these equations
initial water weight=.9*4=3.6
final water weight=.9*4-3=.6
but you stated x/w=4/3 earlier and if that's a ratio of x and w, how can we take 4 and 3 as their value?

Before evaporation : Salt 10% = 10 litres -------- Water 90% = 90 litres.

After evaporation :

The same 10 litres (10 %) of salt now counts for 40 % salt by weight.

We can calculate : 40 % of x = 10 litres

x = 25 litres

This means now we have 25 litres of solution and 75 litres of water has been evaporated.

Or we can say, If we have 25 litres of the same solution, then we can count 10 litres as 40% of the solution.

Now in 25 litres we have

10 litres salt and 15 litres of water

Initial water quantity : Final water quantity

90 : 15

6 : 1

E.