12 defective pens are accidently mixed 132 good ones. It is not possible to just look at pen and tell whether or not it is defective. One pen is taken out at random from this lot. Find the probability that the pen taken out is good one.
Total numbers of elementary events are: good pen + defective pens = 132 + 12 =144
Let E be the event of taking out a good pen
The numbers favourable events are: 132 being the good pens
∴ P (good pen) =P (E) = 132 /144 = 11/12