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Q.

A lot of 20 bulbs contain 5 defective ones. Suppose the bulb drawn from the lot is found to be non-defective and put back in the lot. Another bulb is drawn. Find the probability that it is defective.

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a

15 19  

b

3 4  

c

1 13  

d

1 4   

answer is D.

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Detailed Solution

Given that in a lot of 20 bulbs, it contains 5 defective ones and a bulb drawn from the lot is found to be non-defective and put back in the lot.
We need to find the probability that the next bulb drawn is defective.
Probability of an event E is defined as:
P(E)=Number of favorable outcomesTotal number of possible outcomes
There are 20 bulbs in a lot. As a non-defective bulb was drawn initially and was put back, the lot still has a total of 20 bulbs.
Therefore, the total number of possible outcomes is 20.
Defective bulbs are 5.
Hence, the number of favorable outcomes is 5.
Let E   be the event that the bulb drawn is defective.
Now, the probability of getting a defective bulb is:
P(E)= 5 20 P(E)= 1 4  
Thus, the probability of getting a defective bulb is 1 4  .
Therefore, option 4 is correct.
 
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