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Probability question - it draw me mad

Discussion in 'Private tutors' started by Carlaxyz, Sep 30, 2020.

  1. Carlaxyz

    Carlaxyz New commenter

    I need your help with one probability question, I am refreshing my maths to comeback to teaching and today is probability day, I have one question which I am not able to do and I am not sure why. Question is from old book by Brian Speed - higher mathematics for GCSE-

    the full question :

    Ann has a bargain box of tins. they are unlabelled but she knows that 6 tins contain soup and 4 contain peaches.

    a) she opens 2 tins. what is probability that
    (i) they are both soup, (ii) they are both peaches?

    b) what is the probability that she has to open 2 tins before she gets a tin of peaches?
    c) what is the probability that she has to open 3 tins before she gets a tin of peaches?
    d) what is the probability that she get a tin of soup if she open 5 tins?

    question a is fine but b, c, d, are a bit of a problem. I do not give you the answers as I want to see what would be your solutions (maybe the solutions in book are wrong, ) also I am interested why I cannot do one question when I did today something like 100 full exam questions (most of them tricky one) without any problem where this one I cannot even crack it with answers (they in form final solution)
    thank you for any help
  2. gainly

    gainly Star commenter

    b) 6/10x4/9 (1st tin soup, 2nd peaches)
    c) 6/10x5/9x4/8 (1st and 2nd soup, 3rd peaches)
    d) 1 (if she opens 5 tins at least one must be soup)
  3. shakes1616

    shakes1616 Established commenter

    Draw a tree diagram and work it out that way.
  4. Carlaxyz

    Carlaxyz New commenter

    thanks, English is not my first language and even after 15 years living in uk it looks that I still have a trouble with some little things, so I understood that (b) two tins must be opened to have a next one tin with peaches. It looks that understanding a question is the most crucial part of solving a problems.
    it really draw me crazy
    thank you for your time to answer my question

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