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,
