Here’s our first MATH lesson. It is so easy that one night, I wound up showing it to everyone in the pizza restaurant. Well, everyone who would listen, anyway. We were scribbling down the answers right on the pizza boxes with such excitement that I couldn’t help it – I started laughing right out loud about how excited everyone was about math… especially on a Saturday night.
When you do this calculation in front of friends or family, it’s more impressive if you hand a calculator out first and let them know that you are ‘testing to see if the calculator is working right’. Ask for a two digit number and have them check the calculator’s answer against yours.
If you really want to go crazy, you can have math races against the calculator and its operator, just as the Arthur Benjamin video shows. (Only you don’t need to do the squaring of five-digit numbers in your head!) Have fun!
Download Student Worksheet & Exercises
So here’s the deal:
11 x 23 = ?
Take the 23 and spread it apart, so it looks like “2 3” with a space in the middle. Now add 2 + 3 to get: 2 + 3 = 5 and insert the 5 into the space. So…
11 x 23 = 253
That’s it. How cool is that??
What about this one? 11 x 45 = ?
Spread apart the 4 and 5, then insert the sum of the 4 and 5 in between to get:
11 x 45 = 495
Whoa…!!! This stuff is soo cool! But wait a second… you hit a speed bump when you try this one:
11 x 86 = ?
It seemed a bit ridiculous to get: 11 x 86 = 8146. The answer should be a 3-digit number, not 4! So, check again and then find that you need to ‘carry’ the one to the first digit, so it becomes:
11 x 86 ==> 8(14)6 ==> = 946
And of course, if you can do 86, you have to give 99 a try:
11 x 99 ==> 9(18)9 ==> 1,089
Now you give yourself a few different numbers to try. Check your answer with a calculator!
Tell me how YOU think this works in the comment field below!
Exercises
- 11 x 11
- 11 x 27
- 11 x 43
- 11 x 49
- 11 x 50
- 11 x 67
- 11 x 79
- 11 x 89
- 11 x 92
- 11 x 96