SOLUTION: I have to find two light bulbs from a collection of 12, all but 3 of which are working. If I test each one in turn, what is the probability that I would find two working bulbs. (

Question 1051563: I have to find two light bulbs from a collection of 12, all but 3 of which are working. If I test each one in turn, what is the probability that I would find two working bulbs.
(a) among the three bulbs tested.
(b) when three bulbs have been tested, but not before
Answer by ewatrrr(24785) (Show Source): You can put this solution on YOUR website!
12 bulbs: 9 working, 3 def
p(working = 9/12 = 3/4
testing 3, each one in turn, probability of finding 2 working
(a) among the three bulbs tested (one def)
P(2 out of 3) = (3/12)(9/11)(8/10) + (9/12)(3/11)(8/10) + (9/12)(8/11)(3/10)
(b) when three bulbs have been tested, but not before
P = 1 -P(all def) - P(2 def)
p =
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