Final answer:
The air pressure at 5500 m (18,000 ft) is approximately 50% of the air pressure at sea level.
Step-by-step explanation:
The air pressure at 5500 m (18,000 ft) is around 50% of the air pressure at sea level. This means that the air is less dense at higher altitudes due to a decrease in the weight of the air column above. This decrease in air pressure can affect various physical phenomena, such as boiling points and partial pressure calculations.