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.
?
Puzzle 200
Three builders are laying a brick wall.
Freddie can lay 1,000 bricks in three hours, Jimmy can lay 1,000 bricks in two hours and Kevin can lay 1,000 bricks in six hours.
How long will it take Freddie, Jimmy, and Kevin to lay 1,000 bricks if they all work together?