Answer
$f(x) = 2x$ is a function that is one-to-one and positive for $x \geq 0$.
$f(x) = x/2$ is its inverse.
Work Step by Step
To find the inverse of a function switch $x$ with $y$ and solve.
$f(x) = 2x$
$x = 2f(x)$
$x/2 = f(x)$
$f(x) = x/2$