math
posted by sabir .
In a group of 100 people 40 own a cat, 25 own a dog and 15 own a cat and a dog.
Find the probability that a person selected at random from the group:
a) owns a dog or a cat.
b) owns a dog or a cat, but not both
c) owns a dog, given that he owns a cat
d) does not own a cat, given that he owns a dog.

N(C or D) = N(C) + N(D)  N(C and D)
= 40 + 25  15 = 50
prob(C or D) = 50/100 = 1/2
N(Cat or Dog, not both) = 40 + 25  15 15 = 35
Prob(that event) = 35100 = 7/20
c)
Prob( A  B)  conditional prob
= Prob( A and B)/Prob(b)
prob(dog  cat) = prob(dog and cat)/prob(cat)
= (15/100)/(40/100) = 3/8
d)
prob(cat  dog) = prob(cat and dog)/prob(dog)
= (15/100) / (25/100) = 3/5