Question:
(i) A lot of 20 bulbs contain 4 defective ones. One bulb is drawn at random from the lot. What is the probability that this bulb is defective?
Solution:
GIVEN: A lot of 20 bulbs contain 4 defective ones. One bulb is drawn at random from the lot
TO FIND: Probability that it is defective
Total number of bulbs is 20
(i) Total numbers of bulbs which are defective is 4
We know that PROBABILITY = 
Hence probabilities of bulbs which are defective is 
(ii) Suppose the bulb drawn in case (i) is not defective and not replaced
Then total number of bulbs will be 19.
Total numbers of bulbs which are not defective is 
We know that PROBABILITY =
Hence probabilities of bulbs which are not defective are 