Question

Using a tap, a seven gallon container and a eleven gallon container, can you measure exactly 9 gallons? You may empty the containers down the drain.

Answer 1

We have a container which we fill 9 gallon of water.

  Take 7 gallon full of water,  pour in to  the 11 gallon.  Take again 7 gallons with the 7 gallon container, pour into the 11 gallon container, till it is full.  So 3 gallons are remaining in it now.    Pour it into our container, where you want  9 gallons.

  Empty the 11 gallon container.  Repeat the above procedure two more times.
  Then you get  9 gallons.