You are standing next to an unlimited source of water (the tap in your kitchen). You need precisely 4.6 litres of water. Luckily you found two empty buckets. One can hold 4.3 and the other 4.7 litres of water. How can you measure the correct amount of water using the buckets when neither of them has graduation marks?
Problem #7 from: http://erityistehtavat.puolustusvoimat.fi/cyberchallenge.html