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Answer :
The total numbers of outcomes i.e. pens = 144
Given, numbers of defective pens = 20
\(\therefore \) The numbers of non defective pens
= 144 - 20 = 124
\( P(E) \ = \ \frac{Number \ of \ favourable \ outcomes}{Total \ number \ of \ outcomes} \)
(i) Total numbers events in which she will buy them = 124
So, P (buying) \( = \ \frac{124}{144} \ = \ \frac{31}{36} \ = \ 0.86 \)
(ii) Total numbers events in which she will not buy them = 20
So, P (not buying) \(= \ \frac{20}{144} \ = \ \frac{5}{36} \ = \ 0.138 \)