Do some mathematical mumbo jumbo then SUBTRACT THE YEAR YOU WERE BORN (doh!).

As soon as you plug you birth year into the process you've given all the data to know your birth year. Couple that with the "Have you had your birthday this year" question, and add 1 more for "yes".

I've got another stumper for you:

Pick a number at random from 1 to 100.
Add 35.
Subtract 36.
Add 1, and THAT"S YOUR NUMBER! Amazing, ain't I?

This little bit of calendar math is no different in principle, it's just been ever so slightly disguised with irrelevant hocus pocus.

(Disclaimer: Retired engineer here. Math is my main thing.)

If I give you my birth year, you can figure my age from that.

Select number = x
2x
2x + 5
(2x + 5)50 = 100x +250
add 1766 for November birthday, you now have
100x + 2016

Multiplying the original number by 100 moves it to the left twice, making it the first digit in the resulting 3 digit number. If you subtract your birth year from 2016, you get your age. So you get a three digit number with the first the number you selected and the last two your age.

I don't imagine it would work if you're over 100.

It worked for me.

Lol.

Walk in with an envelope and shake it to show there are several coins inside. How many coins? Tell students:

Make a guess.
Add seven
Double your result
Add four to that result
Divide that result by two
Subtract your original guess
Divide by two.
That's how many coins are in the envelope. Lots of puzzled looks so I start over again with the instructions. Maybe do it a third time. Then have a student open the envelope and find there are 4and 1/2 coins inside. I use a tin snips to cut a coin in half.

Somewhere inside all the calculations it gets you to give away what the trick purports to be guessing. As soon as I got to the part about adding one number if you've had your birthday, that number less one if you haven't had it yet, I knew where it was going.

That anyone with more than fourth grade math can be surprised by this says a lot about the math ignorance of so many.

I just figured most people don't sit down to do the math. Thanks.

is how the give away is buried inside the math.

There are similar things where you're asked to pick a number from 1 to 10, do some manipulations, and the other person then tells you what number you end up with.

I just googled "Pick a number between 1 and 10" and got a bunch of results. Here's a link to a bunch of manipulations you can do, which are briefly explained. http://www.wikihow.com/Read-Someone%27s-Mind-With-Math-(Math-Trick)