An airplane travels 700 miles against the wind in 4 hours, and makes the return trip with the same wind in 2 hours. Find the rate of the wind.
Let v be the speed of the airplane, let w be the speed of the wind (absolute value).
General equation x = vt
In our case,
700 = (v - w) * 4 = 4v - 4w (Eqn 1)
700 = (v + w) * 2 = 2v + 2w (Eqn 2)
Multiply (Eqn 2) by -2
-1400 = -4v - 4w (Eqn 3)
Add (Eqn 1) and (Eqn 3)
-700 = -8w
w = 700/8 = 87.5 miles/hour