Logical Puzzles
Hard
4 min
A person has exactly 7 litres of water but only a 5-litre and a 3-litre measuring jug. How can the person measure exactly 4 litres?
Hint
Use the difference between 5 and 3 litres.
Answer
4 litres in the 5-litre jug.
Explanation
Fill the 5-litre jug and pour into the 3-litre jug, leaving 2 litres. Transfer those 2 litres to the empty 3-litre jug. Fill the 5-litre jug again and pour into the 3-litre jug until it is full. Since the 3-litre jug already contains 2 litres, only 1 litre is transferred, leaving exactly 4 litres in the 5-litre jug.
Step-by-Step Solution
1. Fill the 5L jug.
2. Pour into the 3L jug.
3. 2L remain in the 5L jug.
4. Empty the 3L jug.
5. Transfer the 2L into the 3L jug.
6. Fill the 5L jug again.
7. Pour from the 5L jug into the 3L jug until full.
8. Only 1L transfers.
9. Exactly 4L remain in the 5L jug.
2. Pour into the 3L jug.
3. 2L remain in the 5L jug.
4. Empty the 3L jug.
5. Transfer the 2L into the 3L jug.
6. Fill the 5L jug again.
7. Pour from the 5L jug into the 3L jug until full.
8. Only 1L transfers.
9. Exactly 4L remain in the 5L jug.