Bunuel wrote:

A solution of salt and water is 10 percent salt by weight. After a period of time under pressure and heat, a portion of the water evaporates so that the solution is 40 percent salt by weight. What is the ratio of the initial weight of water to the final weight of water in the solution?

(A) 1 to 6

(B) 1 to 4

(C) 1 to 3

(D) 4 to 1

(E) 6 to 1

We'll use easy numbers to help avoid confusing calculations.

This is an Alternative approach.

Say our solution is 100L. Then there are 10L worth of salt.

After the evaporation, these 10L are now 40% of the solution so the total volume is 10/40% = 10*100/40 = 10*2.5 = 25L

Then we had 100-10 = 90L of water at the start and 25 - 10 = 15L of water at the end.

So the ratio is 90 : 15 = 6 : 1

(E) is our answer.

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