Nov 25, 2017
RAY: I stole this puzzler from a little book by Terry Stickles, and there's a foreword in here by Will Shortz, who, of course, is the puzzle editor of the New York Times and a frequent visitor to National Public Radio.
You've been invited to go on a camping trip in the woods with 30 of your closest buddies, and you've rented a cabin in the woods. You pile into your cars and drive to the cabin. The next morning, everyone gets up and decides that Cookie is going to make homemade pancakes for everyone, but he needs to add to the recipe exactly two gallons of water. So you are sent to the well to fetch two gallons of water with no measuring device. When you get to the well, you discover there are two jugs there. One says 13 gallons, and the other says seven gallons. And your job, if you choose to accept it, is to come back with exactly two gallons of water.
TOM: Can you make two trips to the well?
RAY: No, you can't! They're earthenware jugs! They're very heavy, and you've got a bad ticker, a bad back and a bad front. So you're allowed one trip.
RAY: Here's how you would do it. You take the seven-gallon container and you fill it up and you pour the contents of it into the 13-gallon jug. And then you do that again. And when you do that…
TOM: It's not all going to fit.
RAY: It's not all going to fit. You'll have filled the 13-gallon container, and you will have one gallon left over in the seven-gallon container.
RAY: You put that aside. You pour out the 13-gallon container. So now you have one gallon in the seven-gallon container, and the 13-gallon container is empty. You then take that one gallon and you pour it into the 13-gallon container, leaving room for 12 more gallons. You then fill the seven-gallon container again and pour the entire seven gallons into the 13 gallon container.
TOM: Now you're up to eight in there.
RAY: Now you're up to eight gallons, and you say, "Mmm. Eight. Room for how many more?"
RAY: Five. You then fill the seven-gallon container and you pour all but two gallons in, because there's only room for five in the 13-gallon container.
TOM: And you'll have two gallons left.
RAY: You'll have two gallons left, and if you don't trip and fall on the way back to the cabin, they'll be able to make the pancakes.