Answer:
The answers are in the question.
Explanation:
The wording can be off-putting but the answers are mostly there for you. It says that when the wind was at the tail of the plane, the travel was hastened to 5 hours instead of six. Is the rate of speed with the wind in favor of the plane is r+21, then it stands to reason that traveling AGAINST the wind will cause the rate to drop to r-21. Taking the average of the two travel times (5 and 6 hours each), we get 5.5 hours. Since speed is distance divided by time, the speed of the plane in still air will be d / 5.5
Be sure to label the speed as mph.