Puzzle ZZMQ 

Workings

Hint
Start by filling B.

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
0  5     empty A

This leaves 5 gallons in container B.