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
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.
? 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"
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:
"75 times 11, OK, 7-something-5, 7-12-5, oh crap, a hard one, 8-2-5"
Easy ! one more for luck:
"83 times 11, OK, 8-something-3, 8-11-3, oh crap
, another hard one, 9-1-3"
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
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.
"99 times 11, OK, 10-something-3, 10-8-9"
You can also use similar principles to this to divide
Now, if you do the following practices in your head
right now, there's a great probability
that you'll remember
this rule automatically
63x11 = ?
34x11 = ?
24x11 = ?
78x11 = ?
56x11 = ?
87x11 = ?
95x11 = ?
40x11 = ?
11x11 = ?