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.

Answer 1

Total number of pens = 12 + 132 = 144
Total number of good pens = 132
$\text{Probability}\ \text{ of}\ \text{ getting}\ \text{ a }\ \text{good}\ \text{ pen}=\frac{\text{Number}\ \text{ of} \ \text{favourable}\ \text{ outcomes}}{\text{Number}\ \text{ of }\ \text{total }\ \text{possible} \ \text{outcomes}}$
$=\frac{132}{144}=\frac{11}{12}$