Question

An airplane flies to San Francisco from Los Angeles in 4 hours. It flies back in 3 hours. If the wind is blowing from the north at a velocity of 20 mph during both flights, what was the airspeed of the plane (its speed in still air)?

Answer 1

140 mph

Explanation:

because with added 20mph when the plane goes back the same distance to san francisco in one less hour, the answer is 140 mph

Answer 2

Airspeed in still air: 140 mph

Explanation:

Let s represent the speed in still air. Recognize that the distance traveled is the same in either direction.

Also note that SF is north of LA.

Distance to SF from LA = Distance to LA from SF

(s-20)(mph)(4 hr) = (s+20)(mph)(3 hr)

Then:

4s - 80 = 3s + 60 => s = 60 + 80 + 140 (mph)

The airspeed of the plane was 140 mph in still air. With a tail wind, the plane is faster; with a headwind, slower.