Final answer:
To find the average speed of the plane in calm conditions, you need to calculate the distances traveled with the tailwind and headwind, and then find the average of the two speeds.
Step-by-step explanation:
To find the average speed of the plane in calm conditions, we need to first find the distance traveled. From the given information, we know that the plane took 2.5 hours to travel from Phoenix to San Francisco with a tailwind of 35 mph, and 2 hours and 45 minutes to return with the headwind. Let's first calculate the distance traveled with the tailwind:
Distance = Time * Speed
Distance = 2.5 hours * (Speed of the plane + Tailwind speed)
Similarly, for the return journey:
Distance = 2.75 hours * (Speed of the plane - Tailwind speed)
By solving these equations, we can find the speed of the plane in calm conditions, which will be the average of the two speeds.