Why The Nine Doesn’t Matter
One thing that my grandfather is superstitious about is that the car number must always add up to 7. So when we were buying a car recently, he checked with me if we’d asked the car dealer about this, and what number had been allotted. It was 8179. So I added the digits: 8 + 1 + 7 + 9 = 25 Adding the digits again to reduce to a single number: 2 + 5 = 7 So all good. Then it struck me that even without the 9, the 8 + 1 + 7 added up to 16, which also gave a total of 7. Curious, I then tried it for some other numbers. 2936? 2 + 9 + 3 + 6 = 20 Reducing to one digit: 2 + 0 = 2 And now, just 2 + 3 + 6 = 11 1 + 1 = 2! 1891? 1 + 8 + 9 + 1 = 19 Reducing: 1 + 9 = 10 Reducing to one digit: 1 + 0 = 1 And now, just 1 + 8 + 1 = 10 1 + 0 = 1! So it appears that the 9 does not matter. But why? This is operation modulo (or mod for short, expressed as %) at work. Mod is an operation whose result is the remainder when one number is divided by another. Fo...