Coin question: prob. that 4th head occurs on 10th toss?

[Guest]

A coin is tossed until four heads occur. What is the probability that the fourth head occurs on the tenth toss?

 

[Gene]

If we drop back to the 9th toss the ways that we got three heads is
9!/(6!*3!) or
C(9,3)
Since the head probability is 1/2 that must be multiplied by 2^-9 to get the (3 head, 6 tail) probability.
The 10th toss head probability of 1/2 is then multiplied by the 9th toss (3 head, 6 tail) probability.

 

[Guest]

Thank you so much for the post. so is the C(9,3) *2^-9 * *.5
thanks again

khurram