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.)

Easy
Go into the store and tell the attendant that you are prepaying for 13 gallons exactly.  Compute the cost, pay and then let them work it out.  The pump won't let you go over then. :)

Hard
Fill the 11 gallon tank.
Empty the 11 gallon into the 6 and 3 gallon tank.
You now have 2 gallons in the 11 gallon tank and you need another 11 gallons, but don't have an empty container.
Here is where the hard part comes in.
Based on commercial evidence that this is possible, you...

Look into the nozzle and confidently say "Release the Hounds!" and quickly insert said nozzle into the 3 gallon container.
Repeat for the 6 gallon container.
Now you just put the 2 gallons into one of the other containers and simply fill the 11 gallon container again.  11+2=13 :)

Posted by Joshua on 2011-09-02 20:10:36