Final answer:
The expression for the resultant mean length l of the rubber band as a function of W is l = W / k.
Step-by-step explanation:
To find an expression for the resultant mean length l of the rubber band as a function of W, we can consider the equilibrium condition of the system.
The tension force in the rubber band, T, balances the weight force, W.
Using Hooke's law, we can write T = k * l, where k is the proportionality constant and l is the elongation of the rubber band.
By solving these equations, we get l = W / k.