Imagine having a urn with 20 balls, of which 8 are black and the remaining white. You are to draw 5 balls out of the urn. What is the probability that at least three of the balls drawn are white?

Answer 1

prob(white) = 12/20
= 3/5
prob(not white) = 2/5

you want prob that
3 are white or 4 are white or 5 are white
= C(12,3) (3/5)^3 (2/5)^9 + C(12,4) (3/5)^4 (2/5)^8 + C(12,5) (3/5)^5 (2/5)^7

I will do the first term, you do the rest
= .012457 +....

Answer 2

I got .155397 but that seems too little a number considering there are more white than black balls?

Answer 3

Would .155397 be correct?