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 Guess your birthday

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Oliver
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Bài gửiTiêu đề: Guess your birthday   30/6/2009, 9:28 pm

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 thousandsof people since 1963 when I learned it. Tell the person (or class) to thinkof 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 4
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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