ISRO | ISRO CS 2017 | Question 3

A bag contains 19 red balls and 19 black balls. Two balls are removed at a time repeatedly and discarded if they are of the same colour, but if they are different, black ball is discarded and red ball is returned to the bag. The probability that this process will terminate with one red ball is

(A) 1
(B) 1/21
(C) 0
(D) 0.5


Answer: (A)

Explanation: Possible outcomes: RR, BB, RB and BR. If two same coloured balls appear both are discarded whereas if different coloured balls appear only black ball is discarded. Black balls will always be used up and 1 red ball will remain at the end regardless of the outcome of the experiment. The probability that this process will terminate with one red ball is always 1.

Option (A) is correct.


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