ritula wrote:

Some part of a 50% solution of acid was replaced with an equal amount of 30% solution of acid. If, as a result, 40% solution of acid was obtained, what part of the original solution was replaced?

(A)

\frac{1}{5}(B)

\frac{1}{4}(C)

\frac{1}{2}(D)

\frac{3}{4}(E)

\frac{4}{5} Source: GMAT Club Tests - hardest GMAT questions

Can sum1 give a simpler explanation?

Solved it by plugging in the option C first :

If 1/2 of the original solution is replaced, that means 50 out of 100 was replaced.

so the replaced part will not have 30% of 50= 15units of acid

the remaining 50 units of the original solution (which was not replaced) has 50% = 25 units of acid

total we have 25+15=40% which is equal to the goal for plugging in ! bingo.. C is the answer !!

Bunuel, please comment if this is an OK approach to solve the question?