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Times 5 Trick





arvicpolvs
Math is everywhere. Calculations are everywhere.

2 x 2 = 4
Simple.

5 x 9 = 45
Hmm.. easy.

what about 26 x 5?
"oww.. gotta check the calculator app"

"..wait"

"It's 130!"

What if you didn't have access to a calculator and you have this problem:
122 x 5 ?

And you need to answer that mentally??

Here's the trick:

* divide 122 by 2.
-- that's 61

* no remainder??
-- just append 0 at the end.. that makes it 610 !

-- if there's a remainder, append 5 otherwise..

now, try 31 * 5 !
cybersa
Nice one.
It will be useful to all maths haters. Wink
Bikerman
And to do 11 is no more difficult.

Let's say 43*11 - easy peasy - 473
or 35*11 - equally piffling - 385

How?
Simple add the digits of the multipland and stick the result between the digits.
So: 44 * 11 (4+4=8, stick 8 between the digits = 484)

So what if the digits sum to >9 ?
No problem - follow the normal rules of arithmetic
49*11 = (4+9 = 13) stick 13 between 4 and 9 as follows:

4 - 9 +
1 3 -
----
5 3 9
(that is, 4 blank 9 plus 1, 3 blank - ie summing the left column with the remainder between the digits)

It is extremely easy with a bit of practice, so I recommend that if you are still struggling then you should try the following, and by the end you will be able to multiply by 11 to almost any size number in your head.....

a) 12*11=? b) 21*11=? c) 33*11=? d)45*11=? e) 166*11=? f) 199*11=? g) 12998*11=?
h) 29977*11=? i) 99289*11=?
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