When we throw the ball along the direction of the wind then the net speed of the ball is sum of speed of ball and speed of wind.
[tex]v = v_{ball} + v_{wind}[/tex]
[tex]\frac{140}{2} = 70 ft/s = v_{ball} + v_{wind}[/tex]
Now when wind is blowing opposite to the direction of the ball then net speed is subtraction of ball speed and wind speed
[tex]v = v_{ball} - v_{wind}[/tex]
[tex]\frac{80}{2} = 40 ft/s = v_{ball} - v_{wind}[/tex]
Now if we subtract above two equations
[tex]70 - 40 = 2*v_{wind}[/tex]
[tex]v_{wind} = 15 ft/s[/tex]
so wind speed will be 15 ft/s