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}