Probabilities
posted by Andrey L on .
A bag contains 5 red, 3 green, 12 white and 7 blue marbles. One is drawn at random. With is the probability it is green if it is not red or blue?

This conditional probability
the formula is
P(A│B), read the prob of A given B
= P(A and B)/P(B)
in your case Prob(A) is Prob(green)
B is "not red or blue"
so find Prob(green AND "not red or blue")
and Prob(not red or blue) and sub into the formula 
There'sa100 red 50 green and 50 blue beads in a jar what's the probability that I'll pull out a blue bead