Variable costs: If x is the number of workers, it will take 40/x hours and cost 1600/x to hire the truck while its being filled. It will cost 5x to have x workers stand idle for 30 minutes at $10/hr. Cost per load will be minimized when variable costs are minimized. So you need to minimize 1600/x + 5x. Take the derivative, set it equal to zero, solve for x.

Fixed costs: $400 in labor to fill truck; $20 of truck hire during return trip.