Two fair dice (one red and one green) are rolled. What is the probability that the sum is 5, given that the green one is either 4 or 3?
2 Answers
Probability
Explanation:
Let
Let
Then we want
# P( A | B) = (P(A nn B)) / (P(B)) #
Consider first
# P(A nn B) = (n(A nn B)) / (n(T)) #
Where,
And,
If
#G=3 => R=2 #
If#G=4 => R=1 #
And so
# P(A nn B) = 2/36 = 1/18 #
Now, let use calculate
# P(B) = (n(B))/(n(T)) #
Where
If
#G=3 => R=1,2,3,4,5,6 #
If#G=4 => R=1,2,3,4,5,6 #
And so
# P(B) = 12/36 = 1/3 #
And so we can now calculate:
# P( A | B) = (1/18) / (1/3) = 3/18 = 1/6 #
See other answer for a "proper" discussion of how to evaluate conditional probabilities.
The explanation (below) is simply offered as an alternate, quick-and-dirty way of seeing this result.
Explanation:
If green is
and red is
As can be seen from the table:
and
So the probability is