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.
Hint
How many statements would be true if the culprit was Alex?
Answer
Emery committed the crime.
Reasoning
If we assume that Alex was the culprit, we can see that there would be 6 true statements, but we know there were exactly 5 true statements, so it can't be Alex.
We can repeat this will all five suspects and quickly see that only Emery being the culprit would result in exactly 5 statements being true.
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Puzzle 20
If 5 cats can catch 5 rats in 5 minutes …
… how many cats do you need to catch 100 rats in 100 minutes?