Explanation:
2904/4 = 726 miles per hour (against wind)
3504/4 = 876 miles per hour (with wind)
Assuming the wind stayed constant, the rate of the wind would be the difference between these two values halved
876-726=150
150/2=75 miles per hour (wind speed)
Therefore the rate in still air would be the middle value of the two:
726+75=801 miles per hour