Answer
P(first double occurs on 4th toss) = 5/6 x 5/6 x 5/6 x 1/6
= $(5/6)^{3}$ x 1/6
P(first double occurs on 5th toss) = = $(5/6)^{4}$ x 1/6
P(first double occurs on kth toss) = = $(5/6)^{k-1}$ x 1/6
Work Step by Step
P(first double occurs on 4th toss) = 5/6 x 5/6 x 5/6 x 1/6
= $(5/6)^{3}$ x 1/6
P(first double occurs on 5th toss) = = $(5/6)^{4}$ x 1/6
P(first double occurs on kth toss) = = $(5/6)^{k-1}$ x 1/6
