A man's car runs out of gasoline. His car tank holds exactly 13 gallons. He has three empty unmarked containers which can hold 3 gallons, 6 gallons, and 11 gallons. Using only these containers at the gas station, how can the man bring back exactly 13 gallons?
(He is not allowed to buy over 13 gallons and dispose of the extra.)
I got 12 and 14 gallons, but not 13 yet. This puzzle will keep me occupied for a while.