A merchant has a barrel containing 60 liters of a liquid mixture that is 20% alcohol by volume. How many liters of pure alcohol must she add to the barrel to create a mixture that is 50% alcohol by volume?
36 liters
Initial pure alcohol = 20% of 60 = 12 liters. Let x be pure alcohol added. New total volume = 60 + x. New pure alcohol = 12 + x. We want (12 + x) / (60 + x) = 0.50. 12 + x = 30 + 0.5x. 0.5x = 18, so x = 36 liters.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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