The first thing we must do for this case is to define variables.
We have then:
P: pressure
A: Area of the sole of the shoe
Since the variation is inverse, we have an equation of the form:

We must find the value of k.
For this, we use the following data:
when the shoes have a sole area of 40 in ^ 2 The pressure is 4 lbs / in ^ 2
Substituting values we have:

From here, let's clear k:

Then, the equation is:
Answer:
The equation that relates these variables is: