Question 39410
The equation we use in these problems is D = RT, distance = rate times time.
With the wind, D = 1, T = 3 and his rate is R + w, w being the wind.
Against it, D = 1, T = 4, and his rate is R - w. 
So we have
3(R + w) = 1
4(R - w) = 1
from which we get
(R + w) = 1/3
(R - w) = 1/4
adding equations we get
2R = 7/12
and
R = 7/24 miles per minute = 60(7/24) = 17.5 miles per hour