This remarkable little trick is based on the properties of a particular number (I am not going to tell you which number, because that would spoil your experience). Please watch the video and perform the calculations that I suggest so that you can enjoy the experience. This part of the video is completed by 2:44.

If you wish to understand how the trick works, then please watch the rest of the video. All will be explained! I even suggest ways that you can challenge and teach your children or students using this knowledge ... and hint at other ways in which it can be put to work. If you understand and enjoy my explanation, you will have taken your first steps into the wonderful world of Number Theory!

Number Theory is a branch/aspect of mathematics in which we analyse how numbers work and what makes them behave the way they do. Number theorists are fascinated by prime and composite numbers, divisibility, number types and patterns. A whole world opens up ... and it is fascinating ... much moreso than you might think! I will be sharing more material relating to number theory as I am able.

If you rearrange any number to form a new number, and find the difference between the two numbers (i.e. subtract), the answer will always be divisible by nine. This formed the basis of the coin trick in my last video (https://youtu.be/t4Ai_SqPQ6A).

If you REVERSE a three digit number and find the difference between the new number and the original one, the answer will always be divisible by eleven AS WELL (and, therefore, by ninety-nine).

This REVERSE AND SUBTRACT pattern ensures that the answer conforms to a particular structure, a9b where the numbers a and b add up to nine, i.e., 099, 198, 297, 396, 495, 594, 693, 792, 891, or 990. Reversing this number (b9a) and adding it to the a9b will ALWAYS produce 1089 as I explain in the video. If you are trying this challenge out with someone, you can have a bit more fun by secreting your answer somewhere and producing it as 6801 ... which will produce a little bit of confusion, before you rotate your answer to reveal the true answer of 1089!

Remember that this trick ALWAYS WORKS provided two rules are adhered to:

1. The starting number must not be a palindrome (i.e. it must not read the same in reverse. If such a number is chosen, when it is reversed and subtracted, the difference will be found to be zero! Technically, this is still divisible by 99 (so the number theory pattern/rule still holds), but it is a redundant sort of result, so we exclude it by starting with a number that looks different when reversed.

2. If, after subtracting, your friend ends up with 99, they must consider it as 099, i.e. as a three digit number, and reverse that to form 990 before adding.

Make these rules clear to your friend, and the trick should work every time.

Because I did not want the video to be unduly lengthy, I did not give a FULL explanation of the number theory principles involved. If you have any questions, please ask them in the comments below the video, and I will do my best to answer them for you.

Number Theory is a branch/aspect of mathematics in which we analyse how numbers work and what makes them behave the way they do. Number theorists are fascinated by prime and composite numbers, divisibility, number types and patterns. A whole world opens up ... and it is fascinating ... much moreso than you might think! I will be sharing more material relating to number theory as I am able.

I enjoyed your presentation and no it wasn’t too long. Each subtraction algorithm has its merit as you demostrated, but after learning the “one up and one down” method, I’m employing it because of its speed and ease of usage. Even my wife, who hates mathematics with a passion, thinks it’s too easy. I look forward to your future presentations on both multiplication and number theory. I read an introduction text book some twenty five years ago on number theory by Oystein Ore who taught at Yale for better than twenty years. So in closing, please produce these lectures and the longer the better. Thanks.
Dennis Bell (on a CCM YouTube video about How to Subtract (Large) Numbers Easily)