A lot consists of 144 ball pens of which 20 are defective and others good.

Solution:

GIVEN: A lot consists of 144 ball pens of which 20 are defective and others good

Nuri will buy a pen if it is good but will not buy if it is defective. The shop keeper draws one pen at random and gives it to her

TO FIND: Probability that

(i) She will buy

(ii) She will not buy

Total number of bulbs is 144

(i) Total numbers of bulbs which are non defective is 

We know that PROBABILITY = 

Hence probabilities that she will buy a good pen which is not defective is 

(ii) We know that sum of probability of occurrence of an event and probability of non occurrence of an event is 1.

$P(E)+P(\bar{E})=1$

$\frac{31}{36}+P(\bar{E})=1$

$P(\bar{E})=1-\frac{31}{36}$

$P(\bar{E})=\frac{5}{36}$

Hence probabilities that she will not buy a good pen is equal to $\frac{5}{36}$