the air pressure from the atmosphere measures 0.5atm at an altitude of 18,000 ft. How much pressure is this in pounds per square inch?

7.5 pounds per square-inch

At sea level, one square-inch of any surface has about 15 pounds of air sitting on top of it. At 18,000 feet, that same square inch has only 7.5 pounds per square-inch (psi) exerted on it.

dynamic equilibrium is the answer.

14 days is the half-life of phosphorus-32