Suppose $f$ is a one-to-one, differentiable function and its inverse function $f^{-1}$ is also differentiable. One can show, using implicit differentiation (do it!), that $$(f^{-1})'(x)=\frac{1}{f'(f^{-1}(x))}$$ Find $(f^{-1})'(4)$ if $f(1)=4$ and $f'(1)=2/5$.

$(f^{-1})'(4) =$