Against the wind, a small palne flew 900 miles in 9 hours. The return trip took only 5 hours. What was the speed of the wind ?
Vp- Vw = 900mi / 9h = 100mi/h.
Vp + Vw = 900mi / 5h = 180mi/h.
Eq1: Vp - Vw = 100.
Eq2: Yp + Vw = 180.
Multiply both sides of Eq1 by (-)1:
-Vp + Vw = -100
Vp + Vw = 180
Sum: 2Vw = 80,
Vw = 40mi/h.