Question:
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.
Solution:
Total number of pens = 12 + 132 = 144
Total number of good pens = 132
$P$ (getting a good pen) $=\frac{\text { Number of favourable outcomes }}{\text { Number of total possible outcomes }}$
$=\frac{132}{144}=\frac{11}{12}$