What is the inverse of the function f(x)=4x?

F -1(x) = 4x
If you supply 3 for the function of F so f(3) = 4x then 4 x 3 = 12
The inverse would be 
One way to work out an inverse function is to reverse the operations that f carries out on a number. Here is a simple example. We shall set f(x) = 4x, so that f takes a number x and multiplies it by 4: f(x) = 4x (multiply by 4). We want to define a function that will take 4 times x, and send it back to x. This is the same as saying that f −1 (x) divides x by 4. So f −1 (x) = 1 4 x (divide by 4). There is an important point about notation here. You should notice that f −1 (x) does not mean 1/f(x). For this example, 1/f(x) would be 1/4x with the x in the denominator, and that is not the same as 1 4 x. 

She will need 5/8 bags for each square yard.