Each week, we post a new Challenge Problem for you to attempt. If you submit the correct answer, you will be entered into that week’s drawing for two free Manhattan Prep GRE Strategy Guides.

If 5 liters of water are added to a barrel when it is half full of water, the amount of water in the barrel will increase by .  If x liters of water are then removed from the barrel, the amount of water in the barrel will decrease to  of the capacity of the barrel.  What is the value of x?

See the answer choices and submit your pick over on our Challenge Problem page.