Before you read below, please know that I figured out the answer as I was doing it, but I decided to post it anyway just in case somehow the information could prove useful to another GMAT studier....

i'm addicted to the chart method (also called box method) because it's generally easy to set up. i am having an issue here though. what am i missing?

"How much water needs to be evaporated from a 30 gallon 20% salt solution to result in a mixture of 40% salt?"

(a) 5
(b) 10
(c) 12
(d) 15
(e) 17

Now, I looked at this and immediately realized that in order for the percentage of the salt in the solution to double, (from 20% to 40%) the entire mixture would have to reduce by 50% (1/2) so I got the answer in under 5 seconds. BUT - on the real exam I don't expect to get so lucky. So I tried to set up the chart but I got lost along the way. Please tell me how to complete the chart and which way I should be going to calculate the correct answer. Thanks.

(sorry for all the dashes, just trying to get things to line up and it doesn't let you put in more than one character space after you click submit)

---------------PERCENTAGE------- MIX --------AMOUNT
SOLUTION 1 ------ .2 ----------- 30 -------- 6
SOLUTION 2 ------ .0 ----------- X --------- 0
FINAL SLTN ------ .4 ----------- 30-X ------ 12 - .4X

Ok - so what do I do now?

12 - .4X = 6

6 = .4X (i changed this to) .4X = 6

4X = 60

x = 15

Ah. You know, this is not the first time I've meant to post something and then figured out the answer as I was creating the post. Usually I just delete them, but in the hopes that somehow this information may help another GMAT studier out there - I'm going to post it anyway.