What is the set fee, or initial value, of this function

The set fee would be $15

Explanation:
The set fee is the starting value. This means that it is the value of the y at x = 0 (y-intercept).

To get the set fee, we would first need to get the equation of the line.

Equation of the linear line has the following general formula:
y = mx + c
where m is the slope and c is the y-intercept

1- getting the slope:
we are given two points which are:
(20,25) and (50,40)
the slope = 

The equation now is:
y = 0.5x + c

2- getting the value of the y-intercept:
To get the value of the c, we will use any of the given points, substitute in the equation and solve for c.
I will choose the point (20,25)
y = 0.5x + c
25 = 0.5(20) + c
25 = 10 + c
c = 15

The equation of the line representing the scenario is:
y = 0.5x + 15

Now, we know that the value of the c is the y-intercept which is the initial value of the function at x=0.
In our situation, this represents the set fee.

Hope this helps :)