# Math Is Fun Forum

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## #1 2021-01-07 17:01:23

ericnumberking
Member
From: Vancouver
Registered: 2018-02-06
Posts: 5

### Multiplying Large Numbers in your head

This is a quick way to multiply two-digit numbers by 11 in your head. Created by a banker believe it or not.

Let's say you are trying to multiply 72 x 11.

Which equals to 792

- Split 72 in to two digits (72 becomes 7 and 2).
- Then add these two digits together (7+2=9)
- Now place 9 in the middle of 7 and 2 and what do you get? 792

Pretty sweet eh?

Let's try that with another number . . . 56 x 11
which equals to

- 56. So 5 And 6
- 5 + 6 is 11 (Just carry the one to the first 5. So 5 becomes 6)
- Answer  is (Drumroll . . .) = 616

Try it out!

Last edited by ericnumberking (2021-01-07 20:35:59)

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## #2 2021-03-27 05:33:35

irspow
Member
Registered: 2005-11-24
Posts: 1,055

### Re: Multiplying Large Numbers in your head

Interesting. I think many people have trouble calculating products in their head because of the way we were taught to do it by hand from right to left which isn’t really our customary way of thinking. Because of this teaching style and the way we think multiplying 72x11 may seem awkward. We are taught to multiply 1x72 then add 1x72 and shift it to the left. This isn’t really natural. At least for me it is much more natural to multiply 10x72 and add 1x72. It just feels natural and simple to add 720+72=792 in my head rather than 72+720. Or maybe I am just strange

I am at an age where I have forgotten more than I remember, but I still pretend to know it all.

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