Add your age this year to the last 2 digits of dob

[maggie]

Add your age this year to the last 2 digits of your date of birth and you will get 111.

Everyone is the world will get this number!

Amazing but I don't know how

[trubble]

I need to know why.

[maggie]

So do I - where is Erasmus, he might know!

[beth]

That's really strange.

Waiting for someone to explain.

[beth]

But hold on.

What if I were 10 and had been born in 2001.

That wouldn't work. You'd just have 11.

[maggie]

I don't know but it has to be the age you will be this year (not the age you are now).

This year, July has 5 Fridays, 5 Saturdays and 5 Sundays. This happens
once every 823 years.

And this year we're going to experience four unusual dates.

1/1/11, 1/11/11, 11/1/11, 11/11/11

[beth]

Has to be for people over 10 years old .

a lady at work explained it to me today as ...

"It's just math. Take any age and the birth year, then as you go to the next, you're adding a number and at the same time subtracting a number, so it will always turn out the same."


I guess that makes an odd kind of sense to me, but if someone hadn't put it just that way I never would have figured it out.

[maggie]

Jun 8, 2011 16:54:53 GMT -5 beth said:

Has to be for people over 10 years old .

a lady at work explained it to me today as ...

"It's just math. Take any age and the birth year, then as you go to the next, you're adding a number and at the same time subtracting a number, so it will always turn out the same."


I guess that makes an odd kind of sense to me, but if someone hadn't put it just that way I never would have figured it out.

Ah, thanks to the lady at your work!

[Deleted]

Jun 6, 2011 12:25:35 GMT -5 @trubble said:

I need to know why.

And you shall.

Let,

x = Your age = number of FULL years between your dob and your last birthday.

y = last 2 digits of your birth year = number of FULL years elapsed between your dob and the previous century mark.

General case ( ie most people born between 1900 and 2000)

Evidently,

x + y = 111 ( simply because the current year is 2011)

However next year it would be 112.

Exceptions
People born before 1900 or after 2000
Again the reason is obvious because our usual numerical system is 10 base.

The case cited by Beth fulfils this parameter.

+++++++++++++++++++++++++++++++++++++

Regards.

Prashna

[beez0811]

28 (soon)
83
____
111

[Erasmus]

Prashna's on the right track. It's one of a number of mathematical quirks that look mysterious because there's hidden algebra going on in the background. A common variety is to perform all sorts of arithmetic but finally comes back to either a fixed number or the number you started with. Somewhere along the way, numbers have been made to cancel out. They are all a kind of arithmetic conjuring trick where all the working is a kind of patter that distracts from the real business.

If you know your algebra (or remember it) there are a lot of easy ways to multiply numbers with just doubling and adding or subtracting, even if they can be long-winded. They were common in the Middle Ages when working bit by bit with Roman numbers was a chore and people hadn't learnt multiplication tables with these foreign Arab-Indian numbers they were a bit distrustful of. It's easy enough to remember about 15 for a Roman multiplication table compared to 45 for Indian (V: *V=XXV, *X=L, *L=CCL, *C=D, *D=MMD, *M=V with a line over it and so on) but adding the partial results up is a nightmare!

If I remember:
*3 - double and add one;
*4 - double twice;
*5 - halve and shift left (or stick a 0 on the end if even, a 5 if odd);
*6 - *5 and add original;
*7 - (7 always a bugger!) *5 and add *2;
*8 - double and either double again or subtract from original with a 0 on the end;
*9 - stick a 0 on the end and subtract;
*11 - stick a 0 on the end and add

And so on.

A nice little bit of algebra that can come in useful is
(a²-b²) = (a+b)·(a-b)

So? Should you have the misfortune to need to multiply two awkward numbers together, it can help if there's something in the middle easy to square and then only needs to subtract the difference (17*23= 391 = 20²-3²), and it works the other way, that should you ever need the square of something like 17, it's easier to to add something to get a nice number like 20, multiply that by subtracting the same and add the square of the adjusting number - (17² = (17+3)·(17-3)+3²= 20*14+3*3 = 140+9 = 149).

If I remember, -e^πi=1 where e and π are irrational and i is the imaginary √-1 - but it all comes out as just plain 1.