math probability
posted by joe on .
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.

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. 
P(b and 2)

help me is suck at math and probability is very hard