Question

$12$ defective pens are accidentally 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. Determine the probability that the pen taken out is good one.

Answer 1

Total no. of pens is $144$ $(132$ good and $12$ defective ones$)$

No. of good ones is $132$

Therefore, $132$ times out $144$ a good pen is taken out.

Let $E$ be the event of taking a pen out.

We know that, Probability $P\left(E\right)$ $=\frac{\text{(No.of favorable outcomes)}}{\text{(Total no.of possible outcomes)}}$$=\frac{132}{144}$$=\frac{11}{12}$

Therefore, the probability that the pen taken out is good one $=\frac{11}{12}$