You have no doubt seen the sales-mix concept in action by observing recent trends in gasoline sales. The old-fashioned gas station has rapidly given way to the convenience store that also sells gasoline. Or in some cases, gas stations might be a partner with, say, Subway, BurgerKing, or McDonald's. In still other instances, the convenience store might also sell fresh produce and flowers, as well as provide services such as a post office, pharmacy, and laundry. Y j What's the reason behind the change?