Porch Potatoes

Jun 21, 1996

RAY: I was struck by the fact that most kids are out of school, or are about to be out of school, for the summer. And lest their little brains turn to mush, I thought that this would be a good puzzler for school kids to ponder. Here it is: Potatoes are 99 percent water and one percent what? Potato. So say you take a bunch of potatoes, like 100 pounds of potatoes and you set them out on your back porch to dry out.

TOM: Yeah, when they are dry they should weigh about a pound.

RAY: Well, not drying out completely. And as they dry out the water begins to evaporate. And after a while, enough water has evaporated so that the potatoes are now 98 percent water. If you were to weigh those potatoes at that moment...

TOM: They'd be lighter.

RAY: Yes, how much lighter? That's the question. Now you can solve this puzzler algebraically, and I invite our school age children to do this, and anyone else who wants to do this. Now if you don't solve it algebraically, you are going to get the wrong answer.

TOM: Really?

RAY: Wait - what's your answer, off the top of your head?

TOM: 99 pounds.

RAY: You are wrong.

Answer: 
RAY: Now, unencumbered by the thought process as usual, my brother guessed 99 pounds.

TOM: Yeah.

RAY: Now, when I guessed, off the top of my head, I guessed about 90 pounds.

TOM: 'Cause it just feels right.

RAY: Yeah. Now if you do the math, 1 percent of 100, which is what the potato is, is one pound. As we told you, that's 1 percent. So 2 percent, when its 98 percent water, two percent of the new weight of the mass is still going to be equal to that one pound, and 2 percent of 50 pounds is a pound. So the potato weight is now 50 pounds, not 100.

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