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}