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Probability

Question
CBSEENMA12033737

A and B throw a die alternately till one of them gets a ‘6’ and wins the game. Find their respective probabilities of winning if A starts first.

Solution

Let S denote the success (getting a '6') and F denote the failure (not getting a '6').
therefore space space space straight P left parenthesis straight S right parenthesis space equals 1 over 6 comma space space straight P left parenthesis straight F right parenthesis space equals space 5 over 6
P(A wins in the first throw) = P(S)= 1 over 6
A gets the throw,  when the first throw by A and second throw by B result into failures. Therefore,
              straight P left parenthesis straight A space wins space in space the space third space throw right parenthesis space equals space straight P left parenthesis FFS right parenthesis space equals 5 over 6.5 over 6.1 over 6 space equals open parentheses 5 over 6 close parentheses squared space open parentheses 1 over 6 close parentheses
straight P left parenthesis straight A space wins space in space the space 5 th space throw right parenthesis space equals space straight P left parenthesis FFFFS right parenthesis space equals space open parentheses 5 over 6 close parentheses to the power of 4 space open parentheses 1 over 6 close parentheses space and space so space on. space
Hence space straight P left parenthesis straight A space wins right parenthesis space equals space 1 over 6 space plus open parentheses 5 over 6 close parentheses squared space open parentheses 1 over 6 close parentheses space plus space open parentheses 5 over 6 close parentheses to the power of 4 space open parentheses 1 over 6 close parentheses plus.... space equals space fraction numerator begin display style 1 over 6 end style over denominator 1 minus begin display style 25 over 36 end style end fraction space space space space space open square brackets because space space straight S space equals space fraction numerator straight a over denominator 1 minus straight r end fraction close square brackets
                             equals space 6 over 11
P(B wins) = 1 - P(A wins) = 1 minus 6 over 11 space equals 5 over 11.

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