There are several information's already given in the question. Based on the information's provided, the answer can be easily deduced.
Distance covered by the hang glider = 303 miles
Time taken by the hang glider for covering the distance = 8 1/2 hour
= 17/2 hours
Then
Average rate of flight = Distance/ Time
= 303/(17/2) miles per hour
= (303 * 2)/17 miles/hour
= 606/17 miles/hour
= 35.65 miles/hour
I hope the procedure is clear enough for you to understand.
