67. Fake Crazy Math Skills

class: tweener | impressiveness: 3/5 | factors: race, fake talent, a lot of process | requires: stuff to write with | watch full episode

Audio Commentary:

(00:01:30)

The Effect: Without letting you see his work, a friend jots down a long column of 10 numbers. The numbers start in the single digits, but go well into the triple digits. The paper is turned over, and your friend is given a calculator. The challenge is simple: once you flip back over the paper so both of you can see the numbers, the race begins to add them all together. He gets to use his calculator, but you have to add them all up in your mind.

Incredibly, when the race starts, you’ll shout out the correct answer in less than 5 seconds and without any help from a calculator.

The Secret: The random column of numbers is generated in a specific way. Tell him to start the column with any two single-digit numbers. Generate a third number by adding the first and second numbers together. Add the second and third number to create a fourth; add the third and fourth to create a fifth, and so on until you have your column of ten numbers.

Creating all the numbers this way will create a Fibonacci-like sequence of numbers, which will have special properties. Even though the first two single-digit numbers are truly random, and nobody could have predicted in advance what numbers will make up the column, you’ll be able to use one of these properties as a shortcut to get the total instantly.

As soon as the race begins, look for the seventh number down. Multiply that number times 11, and you’ll have the total for the entire column.

No matter what the first two starting numbers are, the seventh one down will always be a two-digit number, so you’ll need to learn a trick to quickly multiply it by 11 in your mind.

How to multiply 2-digit numbers times 11:

• 

  step 1: in your mind, split apart the first and second digit and create a blank spot in-between.

• step 2: fill the blank spot with the first and second digit added together.

• step 3: if the first and second digit add up to be a single digit number, you’re done. That 3 digit number is the total of the entire column.

• step 3a: if the first and second digit add up to be a two-digit number, move the tens digit over to the first digit of your grand total, and just keep the second digit where it is.

A huge thanks goes to magician Curt Anderson for showing me this classic! Check him out at magicurt.com