math probability

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this is a probability question...

Suppose you are asked to choose a whole number between 1 and 13 inclusive.
(a) what is the probability that it is odd?...7/13
(b) What is the probability that it is even?...6/13
(c) what is the probability that it is a multiple of 3?...4/13

(d) what i the probability that it is odd or a multiple of 3?.. This is where i am stumped. the book says the answer is 9/13 but i cant find it.

  • math probability -

    the first 3 are correct

    for d)
    the odds are 1,3,5,7,9,11,13
    multiples of 3 are 3,6,9,12

    now the numbers which are either odd OR a multiple of 3 are 1,3,5,6,7,9,11,12,13
    or nine of them
    so prob = 9/13

    There is a formula which says
    Prob(A or B) = Prob(A) + Prob(B) - Prob(A and B)
    = 4 + 7 - 2 = 9

    notice that the 3 and 9 are in both sets, so if we just add up the count, we counted them twice, so we have to subtract one of the counts.

  • math probability -

    P(b and 2)

  • math probability -

    help me is suck at math and probability is very hard

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