No. of good pens = 144 – 20 = 124
No. of detective pens = 20
Total no. of possible outcomes =144 (total no. of pens)
(i) For her to buy it the pen should be a good one.
So, let E = event of buying a pen which is good.
No. of favourable outcomes = 124 (124 good pens)
Probability, P(E) = Number of favourable outcomes/ Total number of outcomes
P(E) = 124/144 = 31/36
(ii) Now, Let
¯
E
= Event of she not buying a pen as it was a defective one.
Therefore, the probability that she will not buy = 5/36.