A lot consists of 144 ball pens of which 20 are defective and the others are good. The shopkeeper draws one pen at random and gives it to Sudha. What is the probability that (i) She will buy it? (ii) She will not buy it ?

Step 1: Analyse the given question
GIVEN: Total number of pens n(P) = 144.
Total number of defective pens = 20.
So, number of non-defective pens = 144 - 20
= 124.
Step 2: Note the probability that she will buy pens
NOTE: If the pens are non defective then she will buy that pens
Let D be event that she will buy a non-defective pen
possible out comes n(D) = .124.
Total number of outcomes n(P)= 144
Therefore the required probability P(D) = \frac{n(D)}{n(P)}
P(D) = \frac{124}{144}
= \frac{31}{36}
Step 3: Determine the probability that she will not buy pens.
NOTE: If the pens are defective then she will not buy pens
Let S be the event that of getting a defective pen.
possible out comes n(S) = 20.
Total number of outcomes n(P)= 144
Therefore the required probability P(S) = \frac{n(S)}{n(P)}
P(S) = \frac{20}{144}
= \frac{5}{36}