Let the solution be 100ml.

It originally contained 40ml of the solute.
Let's say we removed x ml and replaced it with x ml of 25% solution:

40 - 0.4x + 0.25x = 35

0.15x = 5

x = 100/3 ml

Which is 1/3 of 100ml

Answer B.

francoisph wrote:A certain quantity of 40% solution is replaced with 25% solution such that the new concentration is 35%. What is the fraction of the solution that was replaced?

(A) 1/4

(B) 1/3

(C) 1/2

(D) 2/3

(E) 3/4

Use the answer choices.

The answer choices represent what fraction of the solution should be the 25% solution. Normally we'd start with C because it's right in the middle, but we know that the solution can't be half 40% and half 25% because then the resulting percentage would be (40+25)/2 = 32.5. So we need to use less of the 25% solution. The correct answer must be B or A.

Answer choice B:

Let's use a number for the solution that's divisible by 4 (since we'll need to take 40% and 25% of this number) and 3 (since we'll need to take 1/3 of it).

Say the solution = 300.

If 1/3 is the 25% solution, then (1/3)(300) = 100 of the 25% solution, leaving us 200 of the 40% solution.
.25(100) = 25.
.4(200) = 80.
So the solution contains 25 + 80 = 105.
Since 105/300 = 35/100, the resulting solution is 35%. Success!

The correct answer is B.