Some of the 50% solution of acid was removed and this was replaced with an equal amount of 30% solution of acid. As a result, a 40% solution of acid was obtained. What fraction of the original solution was replaced?
Select the correct option:  A) 1/2  B) 1/3  C) 2/3  D) 1/4  E) 2/5
Let x be the fraction of the original solution that was replaced.   0.5*(1-x)+0.3(x)=0.4   0.2x=0.1   x=1/2   The answer is 1/2.
<result>A</result>
Some of the 50% solution of acid was removed and this was replaced with an equal amount of 30% solution of acid. As a result, a 40% solution of acid was obtained. What fraction of the original solution was replaced?