Let the normal rate of the plane = P
Let the rate of the wind = W
With the wind, the effective rate = (P + W)
Against the wid, the effective rate is (P -W)
And Rate * Time = Distance
So
(P + W) * 4 = 700
(P - W)*5 = 700 simplify
4P + 4W = 700
5P - 5W = 700
Multiply the first equation through by 5 and the second through by 4
20 P - 20W = 3500
20P - 20W = 2800 add these
40P = 6300 divide both sides by 40
P = 157.5 mph
And
4 (157.5) + 4W =700 divide through by 4
157.5 + W = 175 subtract 157.5 from both sides
W = 17.5
