Saturday, 20 June 2015

probability - Throw a coloured die until blue face is on top



A game is played by rolling a six sided die which has four red faces and two blue faces. One turn consists of throwing the die repeatedly until a blue face is on top or the die has been thrown 4 times




Adnan and Beryl each have one turn. Find the probability that Adnan throws the die more turns than Beryl



I tried :
Adnan throws two times and Beryl throws once = 23 x 13



Adnan throws three times and Beryl throws once =49 x 13



Adnan throws three times and Beryl throws twice = 49 x 23




Adnan throws four times and Beryl throws once = 827 x 13



Adnan throws four times and Beryl throws twice = 827 x 23



Adnan throws four times and Beryl throws three times =827 x49



The answer says 0.365



Please help


Answer




When Beryl throws the die once, Adnan can throw it 2,3 or 4 times. The required probability is13Beryl=1(113Adnan=1)Similarly, when Beryl throws the die 2 times, Adnan may throw 3 or 4 times, giving the required probability2313Beryl=2(1(13Adnan=1+2313Adnan=2))and when Beryl throws it 3 times, Adnan throws the die 4 times. The last throw could result in a red face or a blue face. So the probability of this case is232313Beryl=3(232323[23+13]Adnan=4)The sum of these terms yields the required answer.


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