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

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A coin is tossed until four heads occur. What is the probability that the fourth head occurs on the tenth toss?
 
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.
 
Thank you so much for the post. so is the C(9,3) *2^-9 * *.5
thanks again

khurram
 
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