Step-by-step explanation:
I take gas consumption as example.
Consider the followings,
Let M be the gas meter reading per month
A be the fuel adjustment fee
B be monthly maintenance fee
Y be unit cost per Joule
Let X be monthly bill
( 1 meter reading unit = 48 unit of Joule )
Monthly bill payment :
X = Mx48xY + A + B
in function notation, the monthly bill is f(M, Y, A, C)