You can answer this question in several ways. The MRS has nothing to do with the elasticity of demand. That they are the same in this case is mere coincidence. Trevor, Shawn, and Bipul are all buyers of chain saws. Together the three constitute the entire chainsaw market.
Market demand is the horizontal sum of the individual market demands.
40 - .2Q B See the graph below. We are lookin for the value of P such that the absolute value of the product of those two factors is 1. Note that corresponds to the midpoint of the linear market demand curve.