After completing the sequence of steps, how many gallons are left in the five-gallon jug?

• A. 4
• B. 2
• C. 5
• D. 3

Answer: (A)

The key idea is tracking how much water sits in each jug as you perform each pour, since the total amount of water stays the same while it moves between jugs. If the five-gallon jug ends up with four gallons, that means exactly one gallon was moved out of that jug and never returned to it during the sequence. In these puzzles, you start with a known amount in play (often all in the five-gallon jug) and each step transfers a precise amount; the final amount in the large jug reflects the net transfers that affected it. Ending with four gallons is simply a net outflow of one gallon from the five-gallon jug, with no subsequent pours back into it. If nothing had been poured out, you’d still have five; if two gallons had been moved out (and not returned), you’d end with three, and so on.