How do you measure 4 liters of water if you have a 7-liter and a 5-liter bucket?

This method works with any such problem, as long as the two buckets' liter-capacities (or gallon capacities, etc.) have no common factors, or else the common factors are also factors of the amount you're trying to measure. Fill the 7-liter bucket, and empty 5 liters of it into the 5-liter bucket; then dump out the 5 liters. Two liters will remain in the 7-liter bucket; transfer them to the 5-liter bucket. Fill the 7-liter bucket again, and empty enough of the bucket into the 5-liter bucket to fill it. That should only be 3 liters transfered, leaving 4 liters left in the 7-liter bucket. QED.