No. of good pens = 144 – 20 = 24
No. of detective pens = 20
Total no. of possible outcomes = 144 {total no pens}
(i) E ⟶ event of buying pen which is good.
No. of favourable outcomes = 124 {124 good pens}
P(E) = (No.of favorable outcomes)/(Total no.of possible outcomes)
P(E) = 124/144 = 31/36
(ii) Bar E⟶ event of not buying a pen which is bad P(E) + P(Bar E) = 1
P(E)+P(Bar E)=1
P(Bar E) = 1 - P(E)
=1−31/36 = 5/36