**Probability ncert solutions Chapter 15 Exercise 15.1 Question 17**

**17. (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?

**(ii)** Suppose the bulb drawn in (i) is not defective and is not replaced. Now one bulb is drawn at random from the rest. What is the probability that this bulb is not defective?

**Solution**

**(i)**

Let E be the event of drawing a defective bulb.

Total number of outcomes favorable to event E=4 (There are 4 defective bulbs)

Total number of possible outcomes=20 (There are total of 20 bulbs)

P(E)=(Total number of outcomes favorable to event E)/(Total number of possible outcomes)

**(ii)**

One non-defected bulb has been taken out already. Therefore, we now have just 19 bulbs out of which 4 are defective.

Let A be the event of drawing a non-defective bulb.

Total number of outcomes favorable to event A=19-4=15

(There are 15 non-defective bulbs out of 19 bulbs)

Total number of possible outcomes=19 (There are total of 19 bulbs)

P(A)=(Number of outcomes favorable to event A)/(Total number of possible outcomes)