A lot consists of 144 ball pens of which 20 are defective and others good. Nutri will buy a pen if it is good, but will not buy if it is defective. the shopkeeper draws one pen at random and gives it to her. What is the probability that
(i) She will buy it?
(ii) She will not buy it?
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 $\frac{5}{36}$