You need to make a set of five rods for a magic trick, with numbers on them ranging from 1 to 31 inclusive. Then you ask people to pick a number and show you only the top number of whatever rods it is on. From that you have to guess which number the person picked. This has to be done within a few seconds and isn't a matter of remembering what numbers are on them.

What numbers should be on each of the five rods?

Ah yes, that old binary system trick...

Think of the binary system. All the numbers ending in 1 in the binary system go on the first rod. All the numbers with a 1 in the 10's digit go on the second rod. Then all those with a 1 in the 100's digit on the third, and so on. The top numbers should be 1,2,4,8, and 16. Then, simply add the numbers to get the number the person thought of.

I'm probably repeating something, but I hope I brought in at least one new idea.

Posted by Tristan on 2003-11-11 18:59:48