All numbers $1$ to $155$ are written on a blackboard, one time each. We randomly choose two numbers and delete them, by replacing one of them with their product plus their sum. We repeat the process until there is only one number left. What is the average value of this number?
I don't know how to approach it: For two numbers, $1$ and $2$, the only number is $1\cdot 2+1+2=5$ For three numbers, $1, 2$ and $3$, we can opt to replace $1$ and $2$ with $5$ and then $3$ and $5$ with $23$, or $1$ and $3$ with $7$ and then $2$, $7$ with $23$ or $2$, $3$ with $11$ and $1$, $11$ with $23$ so we see that no matter which two numbers we choose, the average number remains the same. Does this lead us anywhere?