Probability 3 Question 12

12. A fair die is rolled. The probability that the first time 1 occurs at the even throw, is

(2005, 1M)

(a) 1/6

(b) 5/11

(c) 6/11

(d) 5/36

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Solution:

  1. Let E be the event of getting 1 on a die.

P(E)=16 and P(E¯)=56

P (first time 1 occurs at the even throw)

=t2 or t4 or t6 or t8 and so on 

=P(E¯)P(E)+P(E¯)P(E¯)P(E¯)P(E)+

=5616+56316+56516+=53612536=511



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