A lot of 20 bulbs contain 4 defective ones. One bulb is selected at random from the lot. What is the probability that this bulb is defective? Suppose the bulb selected in the previous case is not defective and is not replaced. Now one bulb is selected at random from the rest. What is the probability that this bulb is not defective?

Given

Total number of bulbs = 20

Number of defective bulbs = 4

P(E) = (Number of favourable outcomes/Total number of outcomes)

Hence,

Probability of getting a defective bulb = P (defective bulb) = 4/20 = 1/5

Now,

Since 1 non-defective bulb is drawn, then the total number of bulbs left are 19

Hence, the total number of events (or outcomes) = 19

Number of non defective bulbs = 19 – 4 = 15

P (bulb is not defective) = Number of non-defective bulbs/Total number of bulbs = 15/19

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