a. We observe a sequence of independent identical trials with two possible outcomes on each trial, $S$ and $F,$ and with $P(S)=p .$ The number of the trial on which we observe the fifth
success, $Y$, has a negative binomial distribution with parameters $r=5$ and $p$. Suppose that we observe the fifth success on the eleventh trial. Find the value of $p$ that maximizes $P(Y=11$ ).
b. Generalize the result from part (a) to find the value of $p$ that maximizes $P\left(Y=y_{0}\right)$ when $Y$ has a negative binomial distribution with parameters $r$ (known) and $p$