A group of friends go on a 75 km biking trip from Airdrie to Strathmore. On a windless day they can bike 20 km/hr, but today it is windy and the wind is at their backs on the way to Strathmore and it is in their face on the way home. Determine the speed of the wind, if it takes 2 hours less to get to Strathmore than it does to get home.