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Ridiculous question about how you learnt column subtraction

(16 Posts)
janinlondon Fri 19-Sep-08 09:55:43

Really only out of interest - DH (Scottish) and I (Australian) both learnt column subtraction by borrowing by adding the borrowed number to the bottom number in the next column up. Everyone else I encounter seems to borrow by subtracting from the top number in the next column (is this making sense?) and DD has been taught this way too. Is it an English phenomenon, or perhaps a more recent technique? I was thrown into chaos trying to show her four digit column subtraction where the top digit had a series of zeros (I did work it out - not a complete numpty!), but I am sure someone told me the reason why its better to do it the way she's doing it than the way I've always done it. Can anyone explain (in very small words - preferably one syllable - I am QUITE a maths numpty).

TeenyTinyTorya Fri 19-Sep-08 10:01:32

I can't work out how your method would work. If you were taking 24 away from 43, 4 can't be taken away from 3,so you'd borrow 1 from the 4 of 43, and then add it to the 4 of 24? Then you'd have to take 5 away from 3, which just doesn't work. I may have misunderstood you (I'm a maths numpty too!) but I only know how to do it the way your dd has been taught.

witchandchips Fri 19-Sep-08 10:02:13

I don't think there is one right way. The sum 4 -(2+1) is that same as the sum 3 (=4-1)-2 after all. In an ideal world children would be taught both methods and then they could really understand what they were doing. They are not because many primary school teachers are not confident enough with maths themselves. This means that when the children become parents they are more likely to have difficulties explaining the technique to their children and so it will go on. <<witch exits on high horse>>

janinlondon Fri 19-Sep-08 10:04:10

TTT - if subtracting 24 from 43, borrow the one to make the unit column 13-4 - add the one you borrowed to the 2 in the 24, making that column 4-3. ? Out of interest, were you taught in England or somewhere else?

witchandchips Fri 19-Sep-08 10:05:45

=40+3 -(20+4)
=30+13-(20+4) or 40+13-(30+4)
=(30-20)+(13-4) or (40-30) +(13-4)

its the same

TeenyTinyTorya Fri 19-Sep-08 10:08:05

Oh yes, said I was a maths numpty! I forgot you put the borrowed one on to make it 13. And I now realise that it makes no difference whether you take 1 away from the 4 in the tens column or add it on to the 2 underneath. It's hard trying to do that in my head lol! I was home-educated so taught by a mix of English and Scottish people - they all did the same method though.

SaintRobertNewton Fri 19-Sep-08 10:09:30

We were taught 'milk bottles and doorsteps'


units column:
3-4 can't do, so borrow a milk bottle.
becomes 13-4 = 9

tens column:
Repay the milk by putting bottle back on step
so 4-2 becomes 4-3 = 1

answer therefore is 19

This is so much easier to demonstrate with pen and paper than online grin

TeenyTinyTorya Fri 19-Sep-08 10:10:06

Lol thanks witch, I worked it out! My dad did maths at university, and is always frustrated by my maths hopelessness. Latin translation - fine, but anything number related makes my brain pack up.

janinlondon Fri 19-Sep-08 10:13:28

SRN where did you go to school? This is how we were taught too (in Australia in the '60s!!)

witchandchips Fri 19-Sep-08 10:15:17

TTT parsing a sentence in latin needs much the same bits of the brain as working out a maths problem.

TeenyTinyTorya Fri 19-Sep-08 10:23:02

It doesn't work that way for me though. I have a mental block where numbers are concerned, but words I am fine with. I did a Mensa test recently, and the second paper, which is full of number problems, just panicked me!

SaintRobertNewton Fri 19-Sep-08 10:25:17

jan - England, early 70s

WilyWombat Fri 19-Sep-08 10:28:26

Id do it the same way as saint but without the milk bottles and doorsteps!

I did a maths workshop at my sons primary school and they have all sorts of weird ways they do it now. I did try to recreate the formulas on here but it didnt work. Im glad I went because I was teaching him completely the wrong method.

janinlondon Fri 19-Sep-08 10:37:48

WW were they doing "chunking"? Agree that is ghastly!

katebee Fri 19-Sep-08 11:09:34

I learnt the same way as SaintRobertNewton in Herefordshire in the 1970s (although without milk bottles!).

I noticed the subtraction is done the other way round at my DS school and find that very confusing.

WilyWombat Fri 19-Sep-08 11:49:05

Yeah chunking rings a bell but not sure.

First method involved rounding up to the next whole number i.e 84 - 76 done in a line with the pluses on top of arrows above (couldnt recreate it on here)

76 to 80 = +4, 80 t0 84 = +4 therefore 4+4 is 8

Next one 468 - 124

400 60 8
100 20 4
300 40 4 = 344

Apparently so they understand it is 100s 10s and units.

Were about 4 different methods until they ended up with stage 4...doing it the same way as us grin

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