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

Given:

12 defective pens are accidently mixed with 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. 

To do:

We have to find the probability that the pen taken out is a good one.

Solution:

Number of defective pens $=12$

Number of good pens $=132$

Total number of pens $=12+132=144$

This implies,

The total number of possible outcomes $n=144$.

Total number of favourable outcomes(the pen taken out is good one) $=132$.

We know that,

Probability of an event $=\frac{Number\ of\ favourable\ outcomes}{Total\ number\ of\ possible\ outcomes}$

Therefore,

Probability that the pen taken out is a good one $=\frac{132}{144}$

$=\frac{11}{12}$

The probability that the pen taken out is a good one is $\frac{11}{12}$.