Question

How do you find the inverse of `f(x)=x/5+4`?

Answer 1

Say that `f(x) = y`, then isolate `x`.

`y = x/5 + 4`
`y - 4 = x/5`
`5y - 20 = x`

Call `x` `f^(-1)(x)`, or `f(y)` or just `x`, whichever you prefer, I myself prefer `f(y)`. If you've chosen `f(-1)(x)` then you need to change `y` to `x`.

`f(y) = 5y - 20`
`f^(-1)(x) = 5x - 20`
`x = 5y - 20`