In: Math
A fair coin is tossed repeatedly until it has landed Heads at least once and has landed Tails at least once. Find the expected number of tosses.
Let
be the
probability of landing Heads.
The experiments will terminate in
tosses,
1) When the
toss is a
Head preceded by
Tails.
Whose probability is
2) When the
toss is a
Tail preceded by
Heads.
Whose probability is
The above 2 events are disjoint.
The probability that the experiments will terminate in
tosses is

Here
.
Hence,

The expected tosses is

Note that
is the expected value of a Geometric distribution which is 2.
Thus,
