Answer
$g$ and $f$ are inverses of each other
Work Step by Step
$f(x)=2-5x$ and $g(x)=\frac{2-x}{5}$
$f(g(x))=2-5(\frac{2-x}{5})=2-2+x=x$
$g(f(x))=\frac{2-(2-5x)}{5}=\frac{2-2+5x}{5}=x$
As $$f(g(x))=g(f(x))\not=x$$ it follows that $g$ and $f$ are inverses of each other.