Answer
Here are the contents of the containers after each stage:
A B
0 7 fill B
3 4 fill A from B
0 4 empty A
3 1 fill A from B
0 1 empty A
1 0 fill A from B
1 7 fill B
3 5 fill A from B
This leaves 5 gallons in container B after 8 steps.
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Puzzle 4
Complete this grid with the digits 1 to 6 to make the sum correct.
Perform each mathematical operation in the order shown, from left to right, so 1 + 2 x 3 is treated as (1 + 2) x 3 = 9.
Consecutive numbers are not next to each other, there is no ÷ 1, and at no point is a decimal or a fraction used.
Note: this puzzle is not interactive, and the squares cannot be clicked.