 ## 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.

 Birthday Math Trivia Have them take the month            number from their birthday:January = 1, Feb = 2 etc.Multiply that by 5Then add 6Then multiply that total by 4Then add 9Then multiply this total by 5          once again
• Have them add the to that total, the day they were born on. For example, if they were born on June 18th, they add 18.
• Have them give you their total.
• (This last step takes a little practice to do in your head) In your head, subtract 165 from their total, 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!

Now that you've seen some birthday math trivia, here's a birthday math trick you'll probably enjoy.

Here's another math trick / trivia that involves your age. It's really simple to do.

• Multiply your age by 7.
• Multiply that answer by 1443.

Now what do you notice?

For example if you are 12, watch what happens:

12 x 7 = 84

1443 x 84 = 121212

Your age will repeat 3 times in a row!

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