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. Find the probability that the pen taken out is good one.

Answer 1

Total number of pens = 132 + 12 = 144

Number of good pens = 132

Let E be the event of getting a good pen.

∴ P(getting a good pen) = P(E) = $\frac{\mathrm{Number}\mathrm{of}\mathrm{outcomes}\mathrm{favourable}\mathrm{to}\mathrm{E}}{\mathrm{Number}\mathrm{of}\mathrm{all}\mathrm{possible}\mathrm{outcomes}}$
= $\frac{132}{144}=\frac{11}{12}$

Thus, the probability of getting a good pen is $\frac{11}{12}$.