Thursday, August 17, 2006

Guess Your Birthday !

Here's a fun trick to show a friend, a group, or an entire class of people. I have used this fun, mathematical trick on thousands of people since 1963 when I learned it. Tell the person (or class) to think of their birthday...and that you are going to guess it.

Step 1) Have them take the month number from their birthday: January = 1, Feb = 2 etc.
Step 2) Multiply that by 5
Step 3) Then add 6
Step 4) Then multiply that total by four
Step 5) Then add 9
Step 6) Then multiply this total by 5 once again
Step 7) Finally, have them add to that total the day they were born on. If they were born on the 18th, they add 18, etc.

Have them give you the total. In your head, subtract 165, and you will have the month and day they were born on!

How It Works: Let M be the month number and D will be the day number. After the seven steps the expression for their calculation is:

5 (4 (5 M + 6 ) + 9 ) + D = 100 M + D + 165

Thus, if you subtract off the 165, what will remain will be the month in hundreds plus the day!

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5 comments:

BLEU 12 said...

Nice site of tricks. I will have to write some of these down and go from there. Check out The Daily Shave and see if its anything of your interest. I will be back at your site when I get off work today. Have to work..sometimes.

THE STU

DOLCE said...

it is funny !

BetaMax said...

i hate you

yasser said...

cool trick; there must a better way to do it - too many calculations and too long not to mention that people can make mistakes when calculating in their heads; however getting the month in hundreds and then the day is simply beautiful.

Unknown said...

love it ...thanks!!