2022 SSMO Accuracy Round Problems/Problem 6

Problem

Consider an unfair $6$-sided die labeled from $1$ to $6$, such that the probability of rolling a number $m$ is directly proportional to $7-m$. However, if we roll any number $n$, then the probability of rolling a number less than $n$ becomes $0$, and the probability of rolling any number $m$ from $n$ to $6$ inclusive remains directly proportional to $7-m$. The expected number of rolls until a $6$ is rolled can be expressed as $\frac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Find $m+n$.

Solution