**Probability ncert solutions Chapter 15 Exercise 15.1 Question 21**

21. A lot consists of 144 balls pens of which 20 are defective and the others are good. Nuri will buy a pen if it is good, but will not buy if it is defective. The shopkeeper draws one pen at random and gives it to her. What is the probability that

(i) She will buy it?

(ii) She will not buy it?

**Solution:**

**(i)**

Let E be the event that she buys pen.

Number of outcomes favorable to Event E=144-20=124

(There are 124 good pens)

Total number of possible outcomes=144

(There are total of 144 pens)

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

**(ii)**

Let F be the event that she does not buy a pen.

Number of outcomes favorable to Event F=20

(There are 20 defective pens)

Total number of possible outcomes=144

(There are total of 144 pens)

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