Help, oh help!!!

Right, when I was a lass, if you had more than one operation in a sum, the bit you were meant to do first was always put in a bracket. eg:

(10×2) + (6/3)  which you would work out as 20 +2= 22

However, H has a sum that looks like this

10×2 + 6 divided by 3 (can’t find a “divided” sign, but basically it just runs on left to right, with no brackets)

Now my instinct in that case would be to go 10x 2 = 20 ,+6 = 26,  divided by 3, makes 8 and two thirds.

So if there were no brackets, then I would assume you just worked left to right, cumulatively. However, looking at the answer, it is clear that its been worked out by doing it as if there WERE brackets. So my question is – how do you know which parts to band together and do as little sums, if there are no brackets. This has really got me foxed.  Can anyone who learned simple mathematical operations later than 1968 PLEASE explain this to me, before I am forced into sending her to school.  Hannah’s view is that you do the multiplying and the dividing before the addition and subtraction, but when I asked her why, she just said “you just DO”. Which doesn’t really help me.

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11 Comments »

  1. Kirsty said

    Well I was taught the same as you – stuff in brackets first. And I think I would attempt the one without brackets the same as you too, certainly can’t remember anything about certain operations being done first.

    That doesn’t really help does it??

  2. Joyce said

    That;s odd, Kirsty, given, our, ahem, age difference 😉 Did you go to school in Scotland? I’m wondering if it was some bizarre thing they did here in primary schools?

  3. Nic said

    I’d do brackets first if there are any and then just left to right too. Also don’t remember there being any other ‘laws’ about sums like that.

  4. Kirsty said

    rofl – yes totally schooled in Scotland. Wouldn’t surprise me if it was totally different to the rest of the world!!

  5. Ali said

    http://www.purplemath.com/modules/orderops.htm

    H is right!

  6. sarah said

    BODMAS brackets, of, divide, multiply, add, subtract

  7. Joyce said

    Okaaaaaay…….so what are orders (or exponitions in Ali’s version)? I seem to have been off school lifting potatoes or something when this was covered. Though now I do have vague re-collections of Alison and Sarah trying to explain it to me in the more recent past. Though clearly I didn’t take it on board then either

  8. Sarah said

    yeah, just remember BODMAS. I never know what the O is for. Can’t say I’ve ever had to use the thing since school though 😕

  9. HelenHaricot said

    was going to say bodmas too!

  10. asilon said

    O or I for indices is powers of, e.g. 2^3 (2 cubed). It’s just convention, so everyone does things the same way.

  11. tbird said

    what? you think there is a reason why you do things in a certain order? It’s cos the Greeks or the Romans or some other ancient lot did it taht way so it must be right 😉

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