OddThinking

I watched as Ben tried a magic trick on the bar-maid, Grace.

She picked a number between 0 and 63, and he showed her a series of cards with grids of 32 numbers on each of them. She confirmed whether or not the number she thought of was on each of the cards. At the end, he told her the number.

Grace was very impressed; she was dying to know the secret. Ben’s wife, Debbie, was the same – apparently she had threatened divorce, but still he refused to tell her.

I, on the other hand, was dumbfounded. Not by the effect, but by its effect. (Sorry, still haven’t shaken it off.) It was a dumb magic trick. Even if you don’t know the secret, it isn’t hard to see that the six binary answers you give to the six cards are enough to uniquely identify the final number. I actually knew the exact mechanics because I had seen the trick in a show-bag when I was about 10 years old; that should give you an idea of the quality level of this trick. Despite this, Grace and Debbie couldn’t understand it and couldn’t stand not knowing.

Always keen to help out, I found a resolution to the tension a few minutes later.

I called Ben back over to the bar, and I asked him to pick a number that was found on one of his cards.

I asked him to add it to last two digits of the year he was born. Then, if it was even, divide the number by two.

I asked him to add a random number from 1 to 10, and then add the month he was born to the result. Again, it it was even, divide by two.

I asked him to subtract the random number that he had thought up in the recent step.

Finally, add all the digits in the answer together, and add it to the total.

He told me what his final number was, and I have to admit it took me a while to decode the solution.

“Let’s see…. You must be 32 years old,” I said.

Ben was very impressed. He was dying to know how I did that one, and, after getting me to repeat the steps, went away to ponder it.

Meantime, I started negotiating a deal. Grace would persuade Ben to give up the secret to the binary trick to her, in return for me giving up the secret to the age-guessing trick, in return for Grace giving me a free drink.

Grace agreed. Ben agreed. I agreed.

“Well, Ben,” I explained, “The secret is all in the first step.”

“When I picked a number from the card?” asked Ben, perplexed.

“No, that was the second step. The first step was when I asked your wife how old you were.”

For the fellow pranksters, the age-guessing trick works best when the victim has a calculator, so you can throw in sines and cube-roots. Be careful not to request an impossible operation; I’ve been caught asking for the square root of a negative number! Kids are a great invisible source of parents’ ages, but be sure that they are sure.

Oh, and it is best if you can remember the steps so you can repeat them on request. I almost got Grace with the same trick immediately after Ben. She excitedly asked me to repeat the trick on her. I was just starting to when her fellow barmaid, Katie, unhelpfully pointed out that Grace had mentioned her age in conversation with me earlier that night.

Observant people suck.