Posted by **joe** on Thursday, December 18, 2008 at 12:22am.

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 -
**Reiny**, Thursday, December 18, 2008 at 8:29am
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.

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