I’m beginning to realise why maths never made sense to me. I’ve just tried to explain the version of subtraction i have always used and i can’t. It doesn’t make sense. (And here’s where i have to own up to the fact that most of the links Alison left me the other night whirled above my head too.)
I was always taught like this
62
-58
—–
Okay you can’t take 8 from 2 so you borrow 10 from the bottom, the 2 becomes 12 and the 5 becomes a 6. Then it all works perfectly. And it does – only when i tried to explain that, with base 10, i can’t give her any reasonable explaination of what i’m doing. It doesn’t make sense. Why am i doing that?
So i look in the CGP book – IT says:
make the 2 into 12 but borrow from the 6 within the same number. This seems to make a certain amount more sense – the 2 becomes 12, the 6, having lost a 10, becomes a 5. So i think we can do it that way.
But why on earth was i ever taught the other way?
Yep, I always did it borrowing a ten from the top number, which does make perfect sense. Your way is just weird, and no wonder you never understood it! I’d sue your maths teacher if I were you 😉
The first time I read this I thought you had mistyped it. I have never seen your first way of doing it, the latter is the only way I know of subtracting.
Nope, that is absolutely how i was taught, through junnior and senior school. No matter how i look at it now, i can’t see how it’s remotely logical.
You’re right. I’ll sue 😉
Yep that makes more sense to me too:-) Of course it’s the way I’ve always done it.LOL. I’ve never heard it explained with borrowing from the bottom number. Obviously it would work, but a bit more confusing I would think.
I’m with the others. Well not Debbie 🙂
It’s a private school thing.
I think there was some old-fashioned way of doing it, which sounds something like your way – not entirely sure as I’ve never paid much attention to it, and it didn’t seem to make sense – but I thought that had stopped being taught about 10 or 20 years before you were at school – some PNEU aberration? A great example of teaching a method just because it gives you the right answer, rather than teaching you *why* it works.
Still, I don’t think a lot of people really think about why the ‘normal’ way works either. But if you can do it with the rods and cubes, then hopefully it will make sense?
You don’t have enough units to do 2-8, so you swap one of the rods for ten unit cubes, so you can then do 12-8. Then you have 5 rods left to do 5-5 with.
Hope it all starts to get clearer!
I remember having “your” way taught briefly at school as you are giving an extra 10 to the top row so you need to give an extra 10 to the bottom row to make up for it (30-20 being the same as 20-10)but then getting a new teacher who did it the easier way thank goodness!
I don’t even understand the question thats how bad I am at maths
:S
Merry I was taught the same as you, (did’nt know about the other way which seems to be the norm) but to be honest I am sooooo bad at maths that I leave it to DP!
Well, the way the CGP book has it, ie every way but mine, is completely new to me as of today. I feel mildly horrified!
I shall swap anyway, i can’t owrk out the mechanics of how my way works, which seems to me like an excellent reason for dumping it.
Your not alone Merry, ‘your’ way is the way I have always done it (though I was aware of alternatives).
I was taught “your” way too (in Greece in the mid-80s). In fact I’d never heard of the other one which means at least my parents too had been taught that. It never crossed my mind that it didn’t make sense, I just used it…
I was taught the second way too. And I never understood until recently why it worked LOL xx
Oh dear. Well, *I* was taught your way. And er – I seem to have taught Hannah to do the same. Bob does it as well. So some weird sixites aberration will live on through my children, and my children’s children, corrupting all the generations to come with wierd subtration behaviour. Feels pretty cool to me 🙂
You’re a dangerous subversive type all round though Joyce 😉
I (a mathematician) had never heard of “your” method before. Of course, it isn’t “wrong” in the sense that it still gives you the right answer, but the second method is so much more natural.
Imagine you have six 10 pound-notes and two 1 pound-coins, while you have to pay someone 58 pounds. You can’t do that straight away, so you first have to change one note for ten more coins. It’s easy to see now that if you take five 10 notes and eight coins, you’re left with four coins.
My mum was taught your way and taught me that way too but I have to confess I was taught the second way in school and I find it so much easier!