Using a tap, a seven gallon container, and an eleven gallon container, can you measure exactly 9 gallons?
You may empty the containers down the drain.
Puzzle Copyright © Kevin Stone
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Hint
Start by filling the 11 gallon container.
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.
