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Long Subtraction Rules

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idiot101
 Post subject: Long Subtraction Rules
 Post Posted: Wed Nov 19, 2008 5:41 pm 
I forgot a simple rule when it comes to subtraction.


40.20
- 12.21

once you take the first O to the right and make it ten, then 2 becomes 1, at which point this 1 needs to become 11 to subtract 2. From where do you get an additional 1 in order to make this possible. Does the zero to its left become ten, by taking 1 from 4, hence making it 3, then 9?

thanks
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JonathanSchneider
 Post subject:
 Post Posted: Wed Nov 19, 2008 9:32 pm 
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ManhattanGMAT Staff


Posts: 380
That is exactly right!
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Joey Z.
 Post subject: Efficient Calculation
 Post Posted: Fri Dec 05, 2008 9:22 pm 
40.20 - 12.21
= 40.20 - 12.21 + .01 - .01
= 40.20 + .01 - 12.21 - .01
= 40.21 - 12.21 - .01
= 28 - .01
= 27.99
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JonathanSchneider
 Post subject:
 Post Posted: Sun Dec 07, 2008 12:07 pm 
Offline
ManhattanGMAT Staff


Posts: 380
Creative, Joey - I like it.
You could even simplify it a bit:

40.20 - 12.21
= 40.20 - 12.20 - .01 (just break the subtraction into two easy steps)
= 28 - .01
= 27.99
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Guest
 Post subject:
 Post Posted: Mon Jan 26, 2009 2:39 pm 
i have another way that i use, feel free to use if u like it ;)

40.20
-12.21

U start off with 12.21 and start counting to 40.20. 12.21 would take .79 to get to 13. From 13 to 40, it will take 27. and from 40 -> 40.20, it will take .20... then u add up those values

.79
27.00
.20
=
27.99

I can do this very fast, maybe because im used to it.
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StaceyKoprince
 Post subject: Re: Long Subtraction Rules
 Post Posted: Thu Feb 12, 2009 10:59 pm 
Offline
ManhattanGMAT Staff


Posts: 6077
Location: San Francisco
Thanks for the suggestions, guys - keep them coming!

_________________
Stacey Koprince
Instructor
Director of Online Community
ManhattanGMAT
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