Question

Suppose I had 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 first three bulbs tested
(b) when three bulbs have been tested, but not before?

Answer 1

probability that any one bulb is working is
\[\frac{9}{12}=\frac{3}{4}\] and probability it is not working is
\[\frac{1}{4}\]
if you pick three at random the probability that exactly two are working is
\[3(\frac{3}{4})^2\times(\frac{1}{4})\]

Answer 2

bro, the bulb tht is working is 3/12 not 9/12 read the question :)

Answer 3

hmmm
how does a bro interpret the statement
"ALL BUT THREE OF WHICH ARE WORKING"?

Answer 4

wch means 9 are working out of 12. are you sure bro?

Answer 5

How to do the (b) part? Can you please explain?

Answer 6

satellite is wrong

Answer 7

So what's the correct answer?

Answer 8

I think satellite is right for the statement all but 3 is working means 3 are not working