Multiplication by 11
To understand the multiplication by 11 trick, take a look at the following multiplication below:
Did you make the following two important observations?
First, notice that the digits of the number that is multiplied by 11 appear again in the answer
Second, the number in the middle of the answer is always found by adding the digits of the number that is multiplied by 11
This is the basic facts that you have to remember to quickly multiply numbers by 11
Therefore, if you want to use this trick, all you have to do is add the digits of the number and put the answer between the number
Other examples
1) 35 × 11
Just add 3 and 5 to get 8
Put 8 between 3 and 5
The answer is 385
2) 18 × 11
Just add 1 and 8 to get 9
Put 9 between 1 and 8
The answer is 198
3) 43 × 11
Just add 4 and 3 to get 7
Put 7 between 4 and 3
The answer is 473
Now, look at this example 48 × 11
Add 4 and 8 to get 12
Just put 12 between 4 and 8???
No, it does not work like this!
You can still put 2 in between. However, since the number is bigger than 9, you have to carry the 10 represented with a 1 over the 4
1 + 4 = 5. Thus the answer is 528
4) 85 × 11
Just add 8 and 5 to get 13
Add 1 to 8 to get 9
Put 3 between 9 and 5
The answer is 935
5) 78 × 11
Just add 7 and 8 to get 15
Add 1 to 7 to get 8
Put 5 between 8 and 8
The answer is 858

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