If $12$ defective pens are accidently mixed with $132$ good ones. Then It is not possible to just look at pen and tell whether or not it is defective. if One pen is taken out at random from this lot. Then Determine the probability that the pen taken out is good one.

We have,

No. of good pens $=132$

No. of defective pens $=12$

Therefore, the total no. of pens $=132+12=144$

Then we have, the total no. of possible outcomes $=144$

Now, let E be the event of getting a good pen.

So No. of favorable out comes $=132$ ($132$ good pens)

As we know the Probability, P(E) = Number of favorable outcomes/ Total number of outcomes

P(E) $=132/144=11/12$