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Posted 08/17/2010 6:35 pm
I've always been fascinated with numbers. To me they are truly magical.
Here's a little something I picked up when I was really young and used it to
impress my friends at school, and later in life for free drinks at my favorite watering hole!
It revolves around multiplying a 2 digit number by 11. Some of you may know this shortcut,
if not, it's cool and worth learning, I think.
Anytime you multiply a 2 digit number by 11, the middle digit in the answer will be the sum
of the 2 numbers you are multiplying.
For example: to multiply 35 X 11, 3+5 =8, put the 8 in the middle of the 2 digits you are
multiplying and you get 385!
27 X 11 = 297!
15 X 11 = 165!
41 X 11 = 451!
Now, you have to make a minor adjustment when the numbers total more than 9.
For example 87 X 11. Here you add the 2 together to get 15. You add 1 to the first number
you are multiplying, then insert the "ones" like we did before. So, 87 X 11 = 957.
Are you with me? 37 X 11 = 407. 88 X 11 = 968. Got it? Pretty cool right!
If the left digit you are adding 1 to becomes 10, you keep the 10, then insert. 98 X 11= 1078.
However, this isn't the part that won me drinks.
But, you need to have that step down to proceed to the "trick", so make sure you got it.
Have someone (that you can trust to add!) grab a pen or pencil and some paper.
Have them write the numbers 1 - 10 in a column down the left side of the page.
Now, without you looking have them pick 2 random numbers between 0 and 9 and put on lines 1 and 2.
Add those 2 numbers together and put on line 3.
Add lines 2 and 3 together and put on line 4 and so on until they have added lines 8 and 9
and put the answer on line 10.
Tell them you can add the numbers up in your head lightning fast. Have them show you the paper
and before they can even look up, you will have already told them the answer. The total of all 10
of their numbers will always be line 7 times 11!
Let's try one. Let's say they choose 3 & 6:
01. 003
02. 006 (I'm using zeroes for spacing)
03. 009 <- add lines 1 and 2 and put here
04. 015 <- add lines 2 and 3 and put here
05. 024 <- and so on
06. 039
07. 063
08. 102
09. 165
10. 267
When they turn the paper over, look at line 7. OK, it's 63. 63 X 11 = 693. You answer 693!
What! Have them add them up and check you! As long as they have added right, you can't go wrong!
Let's try one when the total is more than 10. Let's say they choose 7 & 8:
01. 007
02. 008
03. 015
04. 022
05. 037
06. 059
07. 096
08. 155
09. 251
10. 406
Glancing at line 7, we see 96. 96 X 11 (9+6=15, carry the one, insert the 5) = 1056! Try it!
Probably boring to a lot of you, but I always thought it was cool.
Stay tuned for more challenges and mathemagics!
Here's a little something I picked up when I was really young and used it to
impress my friends at school, and later in life for free drinks at my favorite watering hole!
It revolves around multiplying a 2 digit number by 11. Some of you may know this shortcut,
if not, it's cool and worth learning, I think.
Anytime you multiply a 2 digit number by 11, the middle digit in the answer will be the sum
of the 2 numbers you are multiplying.
For example: to multiply 35 X 11, 3+5 =8, put the 8 in the middle of the 2 digits you are
multiplying and you get 385!
27 X 11 = 297!
15 X 11 = 165!
41 X 11 = 451!
Now, you have to make a minor adjustment when the numbers total more than 9.
For example 87 X 11. Here you add the 2 together to get 15. You add 1 to the first number
you are multiplying, then insert the "ones" like we did before. So, 87 X 11 = 957.
Are you with me? 37 X 11 = 407. 88 X 11 = 968. Got it? Pretty cool right!
If the left digit you are adding 1 to becomes 10, you keep the 10, then insert. 98 X 11= 1078.
However, this isn't the part that won me drinks.
But, you need to have that step down to proceed to the "trick", so make sure you got it.
Have someone (that you can trust to add!) grab a pen or pencil and some paper.
Have them write the numbers 1 - 10 in a column down the left side of the page.
Now, without you looking have them pick 2 random numbers between 0 and 9 and put on lines 1 and 2.
Add those 2 numbers together and put on line 3.
Add lines 2 and 3 together and put on line 4 and so on until they have added lines 8 and 9
and put the answer on line 10.
Tell them you can add the numbers up in your head lightning fast. Have them show you the paper
and before they can even look up, you will have already told them the answer. The total of all 10
of their numbers will always be line 7 times 11!
Let's try one. Let's say they choose 3 & 6:
01. 003
02. 006 (I'm using zeroes for spacing)
03. 009 <- add lines 1 and 2 and put here
04. 015 <- add lines 2 and 3 and put here
05. 024 <- and so on
06. 039
07. 063
08. 102
09. 165
10. 267
When they turn the paper over, look at line 7. OK, it's 63. 63 X 11 = 693. You answer 693!
What! Have them add them up and check you! As long as they have added right, you can't go wrong!
Let's try one when the total is more than 10. Let's say they choose 7 & 8:
01. 007
02. 008
03. 015
04. 022
05. 037
06. 059
07. 096
08. 155
09. 251
10. 406
Glancing at line 7, we see 96. 96 X 11 (9+6=15, carry the one, insert the 5) = 1056! Try it!
Probably boring to a lot of you, but I always thought it was cool.
Stay tuned for more challenges and mathemagics!
