Anyone good with Math can explain that trick ?

Interesting question. Backtracking through the math, you get: Age + Birthyear = Current year.

If we call the number you chose n, we have the equation:

50(2n+5)+1760

Although it's hard to see, this is the equation for a line... Let's expand it and see!

100n+250+1760

Still not quite clear whats going on, until we reduce the function:

100n+2010

So the number you chose is multiplied by 100, meaning it's impossible for it to relate to the firsts and tens number positions... Now we know that the 100n portion means nothing to the firsts and tens... so we're only left with the year, 2010... we subtract our birth year, and it gives us our age.

As a test of this method, we can actually guess any integer number to begin with as the math makes it irrelevant to the calculation... Also, we can change 1760 to any number ending with 60.

For instance, we're actually free to 'guess' any number, do the calculations, and add 21460860 rather than 1760, and we will still end on the age of the person doing the calculation, so long as he or she is not above age 99. QED.

Hope you find this explanation satisfactory!

1. They choose a number we'll call n.

2. Multiply it by 2: 2n.

3. Add 5: 2n + 5

4. Multiply by 50: 50(2n + 5) = 100n + 250

5. If they've already had their birthday: 100n + 250 + 1760 = 100n + 2010

If they have not already had their birthday: 100n + 250 + 1759 = 100n + 2009.

6. Subtracting their birthday gives 100n + (2010 - birth year) or 100n + (2009 - birth year), depending on whether they have had their birthday or not. The part in parentheses gives their current age. Adding 100n, or 100 times the original number, will give the end result described so long as the person is under 100.

In 2011, for this trick to work, you'd have to add 1761 or 1760 instead of 1760 or 1759...just add one each year.

The first 4 steps if broken down in two parts gives you two component

1) 250 constant (later explained)

2) three digit number followed by 2 zeros 100, 200, 300 till 900 based on the number selected and it will be the same number that you have selected based on the formulae used. Later this adds up with the age derived.

Another trick part is in using 1760+250(constant above) instead of 2010 which is the current year. 1759 for whom BD has not passed.So the current year is already used in the calculation.

A simpler way to understand this is to do the process for all the numbers from 1 to 9 on an excel sheet and compare all of them all together.

Basically, you choose a number Q between 2 and 9 inclusive.

From your steps, I got the following expression:

100Q + (2010 | 2009) - birth year (I used '|' to signify that it could be either 2010 or 2009)

The location of 2010 and 2009 means that this trick is year dependent, that is, it should be updated for next year.

This is because they are essentially computing 2010 (or 2009) minus their year of birth, with a random third digit placed there (given by the 100Q). You could also make Q be equal to 1, but i guess that's just there to confuse you.

Generally, it's a good idea to start with a variable and see what happens to it:

1. I'll pick x

2. 2x

3. 2x+5

4. 100x+250

5. 100x+2010 (As I'm writing this, I'm noticing that I have the current year as my constant...hmm)

6. 100x+20 (Now I have my age as the constant, because the current year minus my birth year is going to be however many years I was alive. The 100x is going to just be the number I started with, in the hundreds place, just like it says.)

You can actually do any number!

oh cool it worked

OMG, COOL!