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.

No. of defective pens = 12

No. of good pens = 132

Total no. of pens = 12 + 132 = 144.

Let E₁ be the event of getting a good pen, then number of favourable outcomes to E₁ = 132

$P(E$1) = \dfrac{\text{No. of favourable outcomes to } E1}{\text{Total no. of possible outcomes}} = \dfrac{132}{144} = \dfrac{11}{12}.

Hence, the probability of taking out a good pen is $\dfrac{11}{12}$.