Velocity (against wind) = 2,688 km / 4 hr = 672 km/hr
Velocity (with wind) = 3,208 km / 4 hr = 802 km/hr
psa means plane velocity in still air
ws means wind speed
psa -ws = 672
psa +ws = 802 We add both equations and get
2 psa = 1,474
plane velocity (in still air) = 737 km/hr
psa +ws = 802 so that means
wind speed = 802 -737 = 65 km/hr
Source:
http://www.1728.org/veloccal.htm
(My website)