Final answer:
The water tank on an airplane is typically filled to 100 percent capacity before departure to ensure enough water for the flight, even if it's less than 4 hours. Other details about engine efficiency and water withdrawal don't address the water tank level.
Step-by-step explanation:
The question regarding how full the water tank would be on a flight less than 4 hours is somewhat ambiguous without more context. However, using common sense and some basic assumptions related to aviation practices, the water tank on an airplane is usually filled to 100 percent capacity or very close to it before departure to ensure there is enough water for all passengers and crew for the duration of the flight, even if the flight is less than 4 hours. Airlines plan for the maximum likely usage to avoid any shortage during the flight.
The efficiency of an engine or percentage of water withdrawal is not directly relevant for determining the water tank level in this context. It's important to note that airplanes are designed to be efficient and effective in their operations, which includes water usage. Airlines often operate with the principle of having percent of the necessary supplies to account for unexpected situations or emergencies. As for the fuel economy comparison and volume of air expansion at altitude, these examples touch on principles of physics, such as pressure and volume relationships, as well as energy efficiency, but do not directly answer the water tank capacity question.