Quickly multiply numbers close to multiples of 10

Go down

Quickly multiply numbers close to multiples of 10

Post  Archis on Fri Dec 26, 2008 8:05 am

The following is a five minute video providing a smarter way of multiplying numbers close to multiples of 10. Testing the new method versus the speed of a TI-83 calculator, you'll find it much faster.

THE BASIC PRINCIPLE:
(10+X)(10+Y)= 100+ 10X+ 10Y+XY= 10(10+X+Y)+XY

In other word, add the units digit of the second number (Y) to the first number (10+X). This will serve as your hundreds and tens digit (that's why it is multiplied by 10 above.) Add the product of the units digit (XY) to 10(10+X+Y.)

For example:
12*13

First it is 10(12+3)= 150
Then add 2*3=6 for,
150+6= 156

This will be highlighted in the video and make things seem easier.
avatar
Archis
Admin
Admin

Posts : 78
Points : 38
Reputation : 4
Join date : 2008-09-29

View user profile http://nysmithmath.forumotion.net

Back to top Go down

Re: Quickly multiply numbers close to multiples of 10

Post  Steve on Tue Dec 30, 2008 10:10 pm

That's cool Archis

i have a question though - how did you get a youtube video onto the forum?
avatar
Steve
The Cool Cube
The Cool Cube

Posts : 85
Points : 4
Reputation : 4
Join date : 2008-10-07
Age : 22
Location : ...here

View user profile

Back to top Go down

Great Video!

Post  BCase on Fri Feb 13, 2009 7:41 am

I was wondering if this could work with numbers like, say, 24 and 25. How would you do it and would this work?

BCase
Novice Poster
Novice Poster

Posts : 14
Points : 2
Reputation : 1
Join date : 2009-02-13
Location : The Land of the Pharaohs

View user profile

Back to top Go down

Re: Quickly multiply numbers close to multiples of 10

Post  Archis on Fri Feb 13, 2009 8:08 am

@BCase: Excellent question you pose

This though only works for base 10. As in it will only work for numbers close to 10, 100, 1000...

Click below for run through of similiar problem except with numbers closer to 100.
Spoiler:
For example:
125*102

Similar steps
(125+2)(100)= 12700
25*2= 50
Answer is 12750

Note how I multiplied by 100 for the first step, because 100 is the closest base 10 to both numbers.

So, sorry, this same method won't work for 24*25. Though if you want to learn how to solve 24*25, pm me and I'll tell you how.

Very Happy
avatar
Archis
Admin
Admin

Posts : 78
Points : 38
Reputation : 4
Join date : 2008-09-29

View user profile http://nysmithmath.forumotion.net

Back to top Go down

Re: Quickly multiply numbers close to multiples of 10

Post  Sponsored content


Sponsored content


Back to top Go down

Back to top


 
Permissions in this forum:
You cannot reply to topics in this forum