Question 37145
Okay.  The governing equation for this kind of problem is RT = D.  Rate times time equals distance.  His rate (with no wind) is given as 5 mph.  Then we set up two equations, one with the wind and one against:

(R + W)T = D
(R - W)T = D

now substitute in what you know

(5 + W)T = 10
(5 - W)T = 4

If we solve the second one for T 

T = 4 / (5 - W)

and then plug it in to the first equation, we get

(5 + W)( 4 / (5 - W)) = 10

Solving this we get W = 15/7 or about 2.143 mph, the wind speed.