Teach me this trick!

To do this trick, you have to know two things:

- How to take a number's Digital Root

- The number 9

Digital Roots

A number's Digital Root is found by adding all of a number's digits together. If the sum is larger than 9, add up the digits in the answer. Keep doing this until you have a single digit number. This number is the number's Digital Root.

For example:

- Let's say your number is 89783.

- Add up all of the digits in your number → 8 + 9 + 7 + 8 + 3 = 35

- The answer has more than one digit, so add up all of the digits in the answer → 3 + 5 = 8

- This answer has only one digit, so we're done!

- The Digital Root of 89783 is 8.

The Number 9

The number 9 is important because whenever you take a number, scramble its digits to make a new number and then subtract the two numbers, the Digital Root of the answer is always 9! Try this with the example problem:

For example:

- The original number was 30198

- The scrambled number was 13089

- The subtraction problem was 30198 - 13089 = 17109

- The Digital Root of 17109 is 1 + 7 + 1 + 0 + 9 = 18 → 1 + 8 = 9

The computer knows that the Digital Root of your complete answer is 9. You removed one digit and gave the rest to the computer. The computer took the Digital Root of the numbers you gave it and then figured out how much it had to add to get to 9. The amount it had to add was the number you crossed out! Let's look at the example again:

For example:

- The subtraction problem was 30198 - 13089 = 17109

- Cross out the 7

- Tell the computer all of the numbers that are left in the answer → 1109

- The computer computes the Digital Root of 1109 → 1 + 1 + 0 + 9 = 11 → 1 + 1 = 2

- The computer computes how much it needs to get to 9 → 9 - 2 = 7

- The computer knows that you crossed out a 7!

Danger:

The computer (and you!) can't tell the difference between someone crossing out a 9 and someone crossing out a 0. That's why the computer sometimes guesses those wrong. Let's look at the example again:

For example:

- The subtraction problem was 30198 - 13089 = 17109

- Cross out the 9

- Tell the computer all of the numbers that are left in the answer → 1710

- The computer computes the Digital Root of 1710 → 1 + 7 + 1 + 0 = 9

- The computer computes how much it needs to get to 9 → 9 - 9 = 0

- The computer knows that you crossed out a 0! → Not correct!

- The subtraction problem was 30198 - 13089 = 17109

- Cross out the 0

- Tell the computer all of the numbers that are left in the answer → 1719

- The computer computes the Digital Root of 1710 → 1 + 7 + 1 + 9 = 18 → 1 + 8 = 9

- The computer computes how much it needs to get to 9 → 9 - 9 = 0

- The computer knows that you crossed out a 0! → Right! But, it's the same answer as crossing out a 9. If someone crosses out a 0, it looks just like they crossed out a 9. Bummer!

Now that you are an expert, would you like to guess the computer's secret number?