How to multiply two digit numbers by 11 in your head
Here's a nifty trick, that will actually impress people to some degree.
If you know your multiplication tables up to 10x10, and can add single digit numbers, you can multiply two digit numbera by 11 in your head instantly.
How to do it:
Take your 2 digit number (say 26).
The answer will always be a three digit number (as long as the first number is less than 90).
The first digit of this number will be the first digit of your originial number (2).
The last digit of this number will be the last digit of your originial number (6).
The middle digit of this number will be the sum of the first and last digits (8).
26*11 = 286.
Sounds cumbersome ? The thought process is something like this:
Let's try 34x11. Your brain will be thinking:
"34 times 11, the answer is 3-something-4, 3-7-4"
34*11=374.
Easy !
Special cases:
If the first and last numbers add up to more than 9, the middle digit of the result is the units of this number, and you must add one to the hundreds (first number) of the result.
This is how your brain will be acting:
example: 75x11.
"75 times 11, OK, 7-something-5, 7-12-5, oh crap, a hard one, 8-2-5"
75*11=825.
Easy ! one more for luck:
example: 83x11.
"83 times 11, OK, 8-something-3, 8-11-3, oh crap, another hard one, 9-1-3"
83*11=913.
Simple !
I use this rule on average twice a week in various situations, it's particularly useful when trying to add 10% to something, common in Australia as we've recently introduced a 10% goods and services tax (let's see $33 + 10% = 33*1.1 = 3-something-3, 3-6-3 $36.30, RIP OFF !).
ADVANCED TOPICS
For numbers over 90, cut straight to the chase, the result will be a four digit number, the first two digits will be 1 and 0, the third digit and fourth digits are found similar to the above.
example: 99x11.
"99 times 11, OK, 10-something-3, 10-8-9"
99*11=1089.
You can also use similar principles to this to divide by eleven.
Now, if you do the following practices in your head right now, there's a great probability that you'll remember this rule automatically for life !:
63x11 = ?
34x11 = ?
24x11 = ?
78x11 = ?
56x11 = ?
87x11 = ?
95x11 = ?
40x11 = ?
11x11 = ?
Good Luck !
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