Is this really surprising?

In general, I think there's way too much irrelevant mail being passed
around on the net, but being a professor of mathematics, I feel that I
have an obligation to provide some education regarding this mail! :-)

In case you're interested, I will explain how it works (most of the time),
why it doesn't work all the time, and how to make it work next year.

First a general comment. I don't want to be spoil-sport, but I don't
understand why people are so amazed by this one. If I tell you my year
of birth and whether I've had my birthday or not this year, then you
don't need a Ph.D. in mathematics to figure out my age!

Please note that this file is from 1998. There seems to be a new
version every year. The 2002 version is about how often you like to
eat chocolate.

> 1. First of all, pick the number of days a week that you would like to
> go out (or eat pizza, whatever).

Pick a number x.

> 2. Multiply this number by 2.


> 3. Add 5.

2x + 5

> 4. Multiply it by 50.

50 (2x + 5) = 100x + 250

The main step so far is that we need the number 100x, the rest is just
there to confuse you.

> 5. If you have already had your birthday this year, add 1748. If you
> haven't, add 1747.

(100x + 250) + 1748 = 100x + 1998 or
(100x + 250) + 1747 = 100x + 1997

> 6. Last step: Subtract the four digit year that you were born.

100x + (1998 - year_of_birth) or
100x + (1997 - year_of_birth)


> You should now have a three digit number:
> The first digit of this was your original number ( i.e. how
> many times you want to go out each week).
> The second two digits are your age!!! It really works.

1998 - year_of_birth or 1997 - year_of_birth is going to give your age,
depending on whether you've had your birthday or not. This is simply the
normal way of computing your age (for 1998).

If we assume that you are less than hundred years old, this will be a
two digit number. If we assume that the number of days you wanted to
go out is greater than zero, then x will be between 1 and 7, so 100x
will be a three digit number ending in two zeros. So 100x + (1998 -
year_of_birth) or 100x + (1997 - year_of_birth) becomes what they

But notice that it doesn't work if you are more than 100 years old (my
grandfather turned 100!), or if you are anti-social and want to go out
zero times each week.

> This is the only year (1998) it will ever work, so spread
> the joy around by mailing this to everyone you know

For another year, all you'll have to do is to change the numbers in step 5.

Helmer Aslaksen
Department of Mathematics
National University of Singapore

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