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Brian Ashton
11-16-2009, 8:53 PM
Just studying for a company finance course and I was working out goodwill long hand... I'm in my forties so for the most part I've used calculators all my life...

Tried to do this:

121 800
-150 000

I sat there for a while and thought I can't believe I've forgotten how to subtract. But when I looked at if for a while I realised I've never tried to subtract a larger number from a smaller one before the old school way, other than inverting it. Is there a process of working out the answer other than inverting it and adding a negative sign.

curtis rosche
11-16-2009, 11:00 PM
google it
get someone else to do it

Eric Larsen
11-16-2009, 11:41 PM
Just studying for a company finance course and I was working out goodwill long hand... I'm in my forties so for the most part I've used calculators all my life...

Tried to do this:

121 800
-150 000

I sat there for a while and thought I can't believe I've forgotten how to subtract. But when I looked at if for a while I realised I've never tried to subtract a larger number from a smaller one before the old school way, other than inverting it. Is there a process of working out the answer other than inverting it and adding a negative sign.

Throw out the ones and the last two zeros. That leaves 218 minus 500. Two hundred minus five hundred is three hundred, but subtract one hundred from that to account for having to subtract another 18 -- so two hundred. That leaves 100 minus 18 which equals 82. Put the two (hundred) at the front then 82, and the two zeros at the end (the ones cancel) and you have 28,200. Insert a minus sign and you're set.

No calculators were used in this answer. :D But this is my convoluted "I suck at math" way of doing things in my head.

Jeff Dege
11-17-2009, 12:21 AM
But when I looked at if for a while I realised I've never tried to subtract a larger number from a smaller one before the old school way, other than inverting it. Is there a process of working out the answer other than inverting it and adding a negative sign.
None that I've ever heard of. Why would anyone waste time devising one, or teaching one, or learning one, when inverting and negating the result works so easily and well?

Tom Veatch
11-17-2009, 12:46 AM
Use Yahoo Answers. That's what all the high schoolers do to get their homework done for them.

I swear, some of the things I see posted in the Mathematics section take 10 times longer to type in the question that it would to work the problem.

Ah, but if they did it themselves, they might learn something. Can't have that, now can we?:(

Lee Schierer
11-17-2009, 8:50 AM
Just studying for a company finance course and I was working out goodwill long hand... I'm in my forties so for the most part I've used calculators all my life...

Tried to do this:

121 800
-150 000

I sat there for a while and thought I can't believe I've forgotten how to subtract. But when I looked at if for a while I realised I've never tried to subtract a larger number from a smaller one before the old school way, other than inverting it. Is there a process of working out the answer other than inverting it and adding a negative sign.

Sure, post it on SMC and get the 10,000 members there to figure it out for you. :D

I did it by hand, but was mentally inverting it. I asked my son in law who is a Math prof to see if he knows anotehr way.

Jerome Hanby
11-17-2009, 9:21 AM
Simple, take the 10's complement of the subtrahend, 150000 = 849999, add 1 to it to get the 10's complement = 850000. Add that ten's complement to the minuend, 121800 + 850000 = 971800. Since there was no final carry of 1, the answer is negative, so we get the nine's complement, 028199, then add 1 to get the 10's complement and we have -28200.

Makes more since with binary and logic chips :D

Brian Kincaid
11-17-2009, 10:14 AM
Simple, take the 10's complement of the subtrahend, 150000 = 849999, add 1 to it to get the 10's complement = 850000. Add that ten's complement to the minuend, 121800 + 850000 = 971800. Since there was no final carry of 1, the answer is negative, so we get the nine's complement, 028199, then add 1 to get the 10's complement and we have -28200.

Makes more since with binary and logic chips :D

Very good Jerome, now do it again with a floating point algorithm.
:eek:
-Brian

David G Baker
11-17-2009, 10:15 AM
Flip the figures, subtract in the normal way then add a minus sign after the answer. :D Nothing fancy but it works.

Jerome Hanby
11-17-2009, 10:42 AM
I'm a c programmer, I don't believe in floating point :eek:. Decimals?, we don't need no stinking decimals!


Very good Jerome, now do it again with a floating point algorithm.
:eek:
-Brian

Bill Arnold
11-17-2009, 11:31 AM
Seems that kids these days don't have to learn anything to get through school. One of our granddaughters was visiting last year. She's now a senior in high school. Something came up about using the 24-hour clock and she didn't have a clue. I tried to show her how simple it is by having her convert 2000 hours to 12-hour clock time by subtracting 12 from 20. She asked for a calculator - didn't have a clue what the answer was! :eek:

Jim O'Dell
11-17-2009, 4:38 PM
I did basically what Eric did...did it in my head. 122000 to 150000 (122 to 150) is 28000 (28), plus 200 to get the 121800 to 122000= 28200.
much faster that way for me. ;) Jim.

Scott Shepherd
11-17-2009, 8:08 PM
Basic subtraction only works for natural numbers, and in this case, it doesn't work. You'd have to use integers for that equation, which would allow you to go negative.

(read it on wikipedia :D )