There are actually two riddles here. The first is the tough one, although once you find the solution (or read it) you will think "that's simple." Difficult, yet with a simple solution - that's the definition of an elegant riddle. The second one is a one of those riddles that "tricks" you.



Determine The CD Weights

You are a manufacturer of blank CDs for small recording studios. You arrive at the post office with ten boxes of them ready to close up and ship out, but you have a problem. Not all ten boxes contain the same CDs. Nine of them contain CDs for music recording, and one is full of Cds that are meant for voice recording, destined for a books-on CD company. The problem is that they look identical, and you forgot to label them.

However, there is one difference. You recall that the "music" Cds weigh 13 grams, and because different CD "blanks" were used, the "voice" Cds weigh 15 grams. You try lifting them to see if you can tell the difference by feel, but you can't.

There is a scale in the post office, but it costs one dollar each time you weigh something. You are trying to keep your costs down, so you want to make as few weighings as possible.That is the riddle. Exactly how do you use the scale as few times as possible to determine which box has the "voice" CDs?

Weighing The Silver

In this riddle, you have 15 small silver bars and a balance scale. One bar is slightly lighter than the others, and you can't tell the difference by feel. The question is this: What's the least number of times you could use the balance scale to determine which silver bar is the light one?

Arriving At Solutions To The Riddles

Believe it or not, in the first riddle, you can determine which box has the "voice" CDs in it by using the scale just once. Think about it for a while, and you'll probably figure this one out.

The second riddle is a trick of sorts. If you randomly put one silver bar on each side of the scale, you could get lucky, and one of them will be lighter. The least number of times you could use the scale to find the light one, then, is one time (watch that wording). The second part of the riddle, then, is to find the least number of times you can use the scale that will guarantee you'll identify the light silver bar.

Why not exercise that brainpower right now? These are not the most difficult riddles.