Anyone good at maths? | The Popjustice Forum

Anyone good at maths?

Discussion in 'Pop & Justice' started by IotV, Mar 17, 2010.

  1. IotV

    IotV Guest

    Hey guys. Generally, I find maths quite easy, but whilst revising for my exam in June, I came across this, and genuinely don't have a clue. Would love if someone could put my mind at rest!

    1. When the price of an apple rises by 12p, I can buy ten fewer for £7.20.
    a) Let the price of an apple before the price rise be x pence.
    Show that

    720/x - 720/x+12 = 10

    b) Find the price of an apple before the price rise.
    A solution by trial and improvement will not be accepted.

    Would be so, so, so thankful if someone could work it out!
  2. I think I can work out a, but I need my calculator for that and I can't find it!
  3. Are we solving for X? I got 24.
  4. IotV

    IotV Guest

    I would love you forever. Calculator on computer?
  5. IotV

    IotV Guest

    Oooh, thanks! Can you show me the working out, please? Want to learn it. Thanks!
  6. Mvnl

    Mvnl Staff Member

    24 it is.
    I sucked at maths, but this wasn't that hard. It was actually rather fun to do something like this again!
  7. IotV

    IotV Guest

    Now I feel stupid! Haha. Can you show me the working out, please? Thanks! And can anyone do b)?
  8. Mvnl

    Mvnl Staff Member

    B is 24.
    After the rise the price 36..

    I'll try to find a way to explain it, but I don't really know how to write it out in an understandable way..
  9. I cheated and used an online quadratic equation calculator since I don't remember how to solve for the roots. But basically I:

    moved the 720/(x+12) to the other side
    720/x = 720/(x+12) +10

    then multiplied everything by x
    720 = 720x/(x+12) = 10x

    then multiplied everything by (x+12)
    720(x+12) = 720x + 10x (x+12)

    720x + 8640 = 720x + 10x^2 + 120x

    10x^2 + 120 x - 8640 = 0
    x^2 + 12x - 864 = 0

    and the roots were (x - 24) (x + 36)

    probably not the easiest way to do it but it works!
  10. IotV

    IotV Guest

    I love you all so much, but especially Wowx4! Thanks!
  11. ^ I used a different method.
    720/x - 720/x+12 = 10
    720(x+12) - 720(x) / x(x+12)=10
    720x + 8640 - 720x / x^2+12x=10
    8640/x^2 +12x = 10
    8640 = 10x^2+120x
    x^2 + 12x - 864 = 0
  12. Thank god the hardest thing I had to do on my GRE's was something like finding the length of the radius of a circle.... Pi R Squared baby!
  13. Everyone seems to have given the answer to b, but not to a. Was that the only part you were stuck on?
  14. I don't really get why a is a question. It's just an algebraic set-up of the information that's been provided. I'm not quite sure how you can show 'getting to' that.
  15. I agree, but that seems to be what they're asking. The best I can come up with is to deliberately force yourself to use a simpler formulation and reduce it to that formula. Maybe something like:

    Let m be the number of apples you used to able to buy for £7.20
    Let n be the number of apples you can now buy for £7.20

    m*x = 720
    => m = 720/x [1]

    n(x+12) = 720
    => n = 720/(x+12) [2]

    Then subtracting [2] from [1] gives:
    m-n = 720/x - 720/(x+12)

    But from the description, m-n = 10 so:

    10 = 720/x - 720/(x+12)

    I've no idea if that's what they're after (it's a little while since my GCSE exams...) so I'd say that's worth checking with a teacher who actually knows the syllabus.
  16. IotV

    IotV Guest

    That's what confused me.
  17. IotV

    IotV Guest

    Is that 'A'? Thanks!
  18. Mvnl

    Mvnl Staff Member

    This reminds me how my Maths teacher always persisted I wrote down all my answers in full sentences, containing the question. So when the question was 'if 2 sides of a triangle are 4 and 6 inches how big is the 3rd side?" I had to write down "if 2 sides of a triangle are 4 and 6 inches, the 3rd side is... because...".
    HATED it. Especially cause he seemed to only require it from me.
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