Question by : How to find conditional probability?
A little help…
A circuit to run a model railroad has 8 switches. Two are defective. If you select 2 switches at random and test them, find the probability that the second one is defective, given that the first one is defective.
I know conditional probability is P(A and B)/P(A), but for some reason, this problem is not making sense to me.
Answer by Billy
This is just a guess but:
P(2nd one of the two is defective|1st is defective)=P(both are defective)/P(1st is defective)=
the denominator is 2/8
the numerator (2/8)*(1/7)
thus, our quotient is 1/7 since the 2/8’s cancel out
We don’t use binomial distribution due to no replacement.
What do you think? Answer below!