2024 SSMO Tiebreaker Round Problems/Problem 2

Problem

Bob is attempting to shoot a 3-point throw. Bob attempts the basket 97 times. Each time, Bob has a $35\%$ chance of making the shot. If $S_1$ denotes the expected number of points Bob will make and $S_2$ the number of points Bob is most likely to make, then $|S_1-S_2| = \frac{m}{n},$ for relatively prime positive integers $m$ and $n.$ Find $m+n.$

Solution

Firstly, the expected number of shots Bob makes is $\left(\frac{35}{100}\right)\cdot 97.$ Now, the number of shots Bob is most likely to make is the largest integer $a$ such that \begin{align*} \left(\frac{35}{100}\right)^a\left(\frac{65}{100}\right)^{97-a}\binom{97}{a}&\geq\left(\frac{35}{100}\right)^{a-1}\left(\frac{65}{100}\right)^{98-a}\binom{97}{a-1}\implies\\ 35(98-a)&\geq65(a)\implies\\ 3430&\geq100a\implies\\ 34&\geq a. \end{align*} So, Bob is most likely to make $34$ shots. Therefore, the answer is \[\left|3\left(\frac{35\cdot97}{100}-34\right)\right| = \frac{3}{20}\implies20+3 = \boxed{23}.\]

~SMO_Team