For any number $x$, we are told that $x\&=7-x$ and $\&x = x -7$. What is the value of $\&(15\&)$?
To solve this problem, I need to apply the definitions of the $\&$ operation twice: once to evaluate $15\&$ and once to evaluate $\&(15\&)$. First, I use the definition that $x\&=7-x$ and plug in $x=15$ to get $15\&=7-15=-8$. Now, I have $\&(-8)$ and I use the definition that $\&x = x-7$ and plug in $x=-8$ to get $\&(-8)=-8-7=-15$. Therefore, the value of $\&(15\&)$ is -15. 