Question:
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. Determine the probability that the pen taken out is good one.
Solution:
GIVEN: 12 defective pens are accidently mixed with 132 good ones. One pen is taken out at random from this lot.
TO FIND: Probability that the pen taken out is good.
Total number of bulbs is 
Total numbers of bulbs which are good are 132
We know that PROBABILITY = 
Hence probability that pen taken out is good is