Probability and Expected Value

西藏之神

係咪M1黎?

A fair die is rolled repeatedly until the first time that 3 of the same number appear. What is the expected number of rolls?

可以用Geometric Distribution黎做..
P(X=x)=(1-p)^(x-1)p
E(x)=1/p

expected no. of rolled until the first time that 3 = 1/(1/6)^3 =216
唔知岩唔岩....