Long Subtraction Rules

[idiot101]

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

[JonathanSchneider]

That is exactly right!

[Joey Z.]

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

[JonathanSchneider]

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

[Guest]

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.

[StaceyKoprince]

Thanks for the suggestions, guys - keep them coming!