Okay, here's another one.
A hard kind.
What is 41 minus 29 ?
9 minus 1 is 8, so make the 8 a 2 because 8 + 2 is 10.
So the answer is either 2, 12, 22, or 32.
What do you think it is??
Well between 29 and 41 is the thirtys, and the thirtys is about ten numbers.
From 30 to 40, there are 11 numbers.
Anyway 29 is just smaller than 30 and 41 is just larger than 39 so the answer is the 12 answer.

Sorry to go a little off topic but I just remembered this trick when I read this post:

98 ÷ 49

Cancel out the 9's leaves:

8 ÷ 4 = 2

Thus, 98 ÷ 49 = 2

John E. Franklin wrote:

Luca, I love it!!  Very sweet that you already did it this way, so awesome!!
And Ricky, Hi Ricky!!
So Luca, how old were you when you figured that method out??  or was it taught to you??

i was always taught the carry the 10 sortof method, but i always found it confusing and harder, and i came up with this method myself while i was in early primary school, and since then i havnt found a better method :p

something usefull when handling money. When giving change you are usually required to subtract varying amounts from nice round numbers such as 2000, 1000, 500 or 100 when doing this, a trick that seems to work for me is to look at the end of the number you are subtracting, and add the first digit that produces 10, after that, its 9 all the way, untill you get to the last, where it becomes one less.

Take a look at this

500000000000000
249580309345985
250419690654015

On top is a nice round number. Below is a number subtracted from it and beneath that is the difference. Note the last digits of the bottom two rows add up to 10, but the rest all add up to 9 except for the first which add up to one less then the last digit at the top left.

However, when you subtract a number that ends in zero, you need to keep writing zero's until you get to a non zero number, then apply the same process for instance

7000000
2346600

first digit is 0, write 0, move to the next, next digit also zero, move to the next, next digit is 6 write a 4 to get 10, all the remaining digits add up to 9 except the last which adds to 1 less then the top number.

And you can see:
7000000
2346600
4653400

note this only works with numbers like 9000, 6000, 20000, 50 and so on. Basicly single digits followed by a bunch of zeros.

It makes sense if you think about how borrowing works when you subtract from a line of zero's.

Last edited by mikau (2006-12-13 17:04:44)

Um, that's just the definition of subtracting things. There's no reason why you'd do it any other way. If you mean you do it all in one go, then you still actually do the subtracting units with a ten carried if needed and then subtracting tens, it's just that it's in your head and quickly.

Gracias!

Devanté - you still havn't explained your method however.

On-topic:
When I was in primary school, sometimes we were made to show the borrowing thing. I HATED when we had to do it. If I've understood this topic correctly, I think I'm doing it the same way you guys. It just seems much more obvious to me.

I think that trick Ricky used is meant to be a joke. If not then do explain!

Seriously? You'd start at 123 and count until you got to 456? That would take you ages and be fairly pointless, considering the triviality of the sum. I'm probably misunderstanding you. Any other way though, and you'd have to do it in more than one step.

mathsyperson wrote:

Seriously? You'd start at 123 and count until you got to 456? That would take you ages and be fairly pointless, considering the triviality of the sum. I'm probably misunderstanding you. Any other way though, and you'd have to do it in more than one step.

A bit like number lines. For 456-123, I would do: 123 + 300 + 33.

Hard to explain.

Yep, that's what I'd do.

Another example;

12253-591

=591+10000+1500+162

Occasionally I might do it a different way. But I have been able to tackle every subtraction problem so far with this method.

For 12253-591, rather than using my method I would just do:

(12253-600)+9

= 11662.

Last edited by Devanté (2006-12-18 18:11:57)

12345-6789

= (+6000)-444
= 5556

Using my method this would only take a second or two...