Some of the 50% solution of acid was removed and this was replaced with an equal amount of 20% solution of acid. As a result, a 35% solution of acid was obtained. What fraction of the original solution was replaced? Choose the correct choice from the following answers
A) 1/3 B) 1/2 C) 2/3 D) 3/5 E) 4/7
Let x be the fraction of the original solution that was replaced.   0.5*(1-x)+0.2(x)=0.35   0.3x=0.15   x=1/2   The answer is 1/2.
<result>B</result>
Some of the 50% solution of acid was removed and this was replaced with an equal amount of 20% solution of acid. As a result, a 35% solution of acid was obtained. What fraction of the original solution was replaced?