Suppose that $g(x)=f^{-1}(x)$. If $g(-15)=0$, $g(0)=3$, $g(3)=9$ and $g(9)=20$, what is $f(f(9))$?
From g(3)=9, we know that f(9)=3.
From g(0)=3, we know that f(3)=0.
f(f(9)) = f(3) = 0.
Other informations are useless ones.
+1245 
