A retailer sells a perishable commodity and each day he places an order for Q units. Each unit that is sold gives a profit of 60 cents and units not sold at the end of the day are discarded at a loss of 40 cents per unit. The demand, D, on any given day is uniformly distributed over [80,140]. How many units should the retailer order to maximize expected profit?

Could you show me the math steps involved here?