Question

A lot consists of 144 ballpoint pens of which 20 are defective and others good. Tanvy will buy a pen if it is good, but will not buy it 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?

Answer 1

Given that total number of pens n(S) = 144.

Given that total number of defective pens = 20.

That means total number of non-defective pens = 144 - 20

= 124.


(1) She will buy:

Let A be event that she will buy a non-defective pen

n(A) = .124.


Therefore the required probability P(A) = n(A)/n(S)

= 124/144

= 31/36.




(2) She will not buy it.

Let B be the event that of getting a defective pen.

n(B) = 20.

Therefore the required probability P(B) = n(B)/n(S)

= 20/144

= 5/36.