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Thread: How Would You Do This?

  1. #1
    Mar 2008

    Question How Would You Do This?

    A packet contains stamps from three different countries.
    The packet contains 4 Spanish stamps, 10 French stamps and 6 German stamps.
    Two stamps are to be removed at random, without replacement.
    Calculate the probability that both stamps will be from the same country.

    Now what i did is a big tree diagram but this was a very long and complicated way to do it, does anyone know an easier way.

    The answer i got was 33/ this what you get?
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  2. #2
    Super Member
    Oct 2007
    London / Cambridge
    I get the same answer as you. I don't see why a tree diagram is necessary. i just considered three separate cases.

    1) getting two Spanish stamps $\displaystyle \frac{4}{20} \times \frac{3}{19} $

    2) getting two French stamps $\displaystyle \frac{10}{20} \times \frac{9}{19} $

    3) getting two German stamps $\displaystyle \frac{6}{20} \times \frac{5}{19} $

    Then just added the probabilities together.

    $\displaystyle \frac{4}{20} \times \frac{3}{19} + \frac{10}{20} \times \frac{9}{19} + \frac{6}{20} \times \frac{5}{19} = \frac{33}{95}$
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