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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: Here are the contents of the containers after each stage:

A      B
0      11          fill B
7      4           fill A from B
0      4           empty A
4      0           fill A from B
4      11          fill B
7      8           fill A from B
0      8           empty A
7      1           fill A from B
0      1           empty A
1      0           fill A from B
1      11          fill B
7      5           fill A from B
0      5           empty A
5      0           fill A from B
5      11          fill B
7      9           fill A from B
0      9           empty A

This leaves 9 gallons in container B.

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