Saturday, July 6, 2019

probability - Throw a fair die repeatedly until I have thrown three sixes



I play a game in which I have to throw a fair die repeatedly until I have thrown three sixes, after which I stop and note the total number of throws. What is the probability that I take six throws?




I did (1/6)3×(5/6)3 and this gave me 0.00268 to 3.s.f.



This doesn't seem right as couldn't this be achieved in many ways by throwing the die to be 6 in different orders? E.g. 6,6,6, non six,non six, non six OR 6,non six, non six, 6, 6, non six



Could anyone explain this?


Answer



In the first 5 throws exactly 2 sixes must be thrown.



The probability of this event is \binom52\left(\frac16\right)^2\left(\frac56\right)^3.




After that a six must be thrown so we end up with a probability of: \binom52\left(\frac16\right)^3\left(\frac56\right)^3


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