If $f(x)=10x,$ what is the equation for generating $x,$ given the random number $r?$

$x=r\sqrt[\phantom{2}]{10}$

$x=10\sqrt[\phantom{2}]{r}$

$x=\sqrt[\phantom{2}]{5r}$

$x=\sqrt[\phantom{2}]{\frac{r}{5}}$

None of the above