You have two buckets. One holds six liters, and the other holds seven liters. There is no way to measure partial bucketfuls accurately, and you want five liters in one of the buckets. You can fill a bucket fully from the water source, and you can pour from one bucket to the other (either the entire contents of the first bucket or until the second bucket is full).
How can you measure five liters?
Submit your answer to Gene Wirchenko <email@example.com>. Your answer should be in the form of a proof. That means to show how your answer must be correct. The deadline is Wednesday, March 22, 2017 at noon Pacific Time. I will post the answer shortly after.