You are standing by the duck pond with two pails. One holds 7 gallons and the other holds 3 gallons. Neither one of them has gallon markings on the side. A duck waddles up and whispers into your ear, “NGWAKGH”, which translates to, “Would you be so kind as to fill your larger pail with exactly five gallons of water?”
How can you get exactly 5 gallons of water in the big pail?
Solution
You are glad you studied Duck two summers ago.
You perform the following procedure:
Fill 3; dump it into 7.
Fill 3; dump it into 7, which now has six gallons in it.
Fill 3; dump one gallon into 7 (you can do this, since it makes 7 full).
The 3 now has two gallons left in it.
Empty 7 completely; pour the two gallons from 3 into 7.
Fill 3; pour it into 7.
The 7 now has five gallons in it.
The duck, staring into your eyes, gently snaps its beak. You interpret this action as a sign of gratitude.
By the way, you are glad not only that you studied Duck two summers ago, but also that you have been doing serious gym workouts. Think about what 7 gallons of water weigh. (1 gallon weighs 8.34 pounds.)