The Trick: Take any calendar. Choose 9 days that form a square like the ones to the right. Add all nine numbers and then divide by the number in the middle.
The Answer: You always get nine.
How It Works: You may be thinking that this is one of those special properties of the number nine, but it’s not. No, this is another excellent example of the use of algebra. Here’s what I mean.
Let’s call the number in the center n. Then the nine numbers would be
To do the trick, I had you add the numbers, so let’s add these numbers.
Why It Works: What we are really doing is averaging. Look back at this step which is the simplified sum of all nine numbers.