Interesting Math Trivia

Here's an interesting math trivia trick to impress a friend, a group, or class. Tell the person (or class) to think of their birthday...and that you are going to guess it.

• Have them take the month number from their birthday:
  January = 1, Feb = 2 etc.
• Multiply that by 5
• Then add 6
• Then multiply that total by 4
• Then add 9
• Then multiply this total by 5 once again
• 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!

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! That's Pretty Cool Math Stuff!