A lot consists of 144 ballpoint pens of which 20 are defective and others good. Tanvy will buy a pen if it is good, but will not buy it if it is defective. The shopkeeper draws one pen at random and gives it to her. What is the probability that (i) she will buy it, (ii) she will not buy it?
Total numbers of elementary events are: 144
Defective pens = 20
Good pens = 144 – 20 = 124
(i) Let E be the event of Tanvy buying the pen
The numbers of favourable outcome are: 124 being only good pens worth buying
∴ P (buy pen) = P (E) = 124/ 144 = 31/ 36
(ii) Let E be the event of Tanvy not buying the pen
The favourable outcomes are: 20
∴ P (not buying a pen) = P (E)= 20/144 = 10/72 = 5/ 36