Using real life quantities, how will you justify that the derivative of constant is 0?

We know that

One way of thinking about the derivative, is as the slope of a function at a given point
for example the function f(x)=3.5
The graph is a line, so the slope is the same at every point. Further, it is a horizontal line. The slope of any horizontal line is zero. Since the graph of any constant function is a horizontal line like this, the derivative is always zero.

It is a percent with money involved too divide and you should have your answer