Birthday Math Trivia

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! Why It Works You can skip down to the next math trivia trick if you don't care to know why it works. We'll explain how this works with some simple algebra. 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 So therefore, if you subtract off the 165, what will remain will be the month in hundreds plus the day! That's Pretty Cool Math Stuff!