Keep in mind that an inverse function is simply the reversal of the x and y coordinates of the parent function. For every (x,y) of the parent function, the inverse function has a point (y,x). With the conceptual part out of the way, finding the inverse function is relatively straight forward. If:
y=5x then the inverse function will be what you have when you solve for x and then reverse the variable labels...ie.
y=5x so
x=y/5 now reverse the variable labels...
y=x/5 this is the inverse function of y=5x or as your problem notation stated:
f^-1(x)=x/5