Tag Archives: numeracy

Math… amazing

Every so often someone will forward me one of these “amazing!” math tricks, and I will of course feel compelled to explain just how outrageously simple the math in them actually is. The latest one going around is even simpler and more obvious than most, and yet people still seem impressed by it:

Take the last two digits of the year you were born, add your age this year, and it will add up to 111. Amazing!

I have to say, I’m kind of amazed that it’s not gobsmackingly obvious to absolutely everyone who can add and subtract two digits. But so many people will do anything to avoid arithmetic, so it seems to have that “magic wand” quality pretty readily.

So OK. Say someone were to send you an email that said “The year you were born plus your age this year equals 2011 – but only this year! Amazing, huh?” Wouldn’t you find that obvious? Now, 2000–1900=100, and you were born in the 1900s (we assume no one under 12 years old got the email), and it’s 2011 now…

Put it another way: if you subtract 1900 from everything, as though 1900 were the year 0, this year would be the year 111; and if you start with the last two digits of your birth year, you’re subtracting 1900, so…

There are some really cool number tricks out there. But you don’t too often see them being passed around in emails, because different people have different definitions of “cool”.

All of this is explainable with simple algebra on the basis that a two-digit number cen be represented as ten times a one-digit number plus another one-digit number, e.g., 49=(4×10)+9.

So for any number 10x+y (e.g., 40+9, where x=4 and y=9), the reverse will be 10y+x (e.g., 90+4), meaning if you add 10x+y (the original number) to it you get 11x+11y (e.g., 40+9+90+4=44+99), and if you subtract the reverse you get 9x–9y (e.g., (40+9)–(90+4)=40+9–90–4), and if you subtract the sum of the digits (x+y, e.g., 4+9) you get 9x (because 10x+y–(x+y)=9x, e.g., 40+9–(4+9)=36). And of course 10x+y+10y+x=11x+11y=(x+y)×11.

So assuming a person of a normal adult age, you can say

1. Take your age (e.g., 49).
2. Add the digits together (e.g., 4+9=13).
3. Subtract that from your age (e.g., 49–13=36).
4. Add the numbers of the resulting number together (e.g., 3+6).

Of course, you want to gussy this up with something fancy. Add in some other calculations to distract. Instead of step 5, maybe say

5. Multiply by the last two digits of the year.
6. The answer is 99. This always works!! But it will only work this year!!! And not again for a hundred years!!!! OMG it’s amazing tell all your friends!!!!11

or, if you think they can handle the math (!), say