(iii) P(r,s) = P(throw r heads before s tails)
"throw r heads before s tails"...
this is equivalent to:
"in the first r+s-1 throws, he throws at least r heads", since:
- if he doesnt, then he has thrown <= r-1 heads, and so he has thrown >= s tails, so not r heads before s tails
- if he...