In the united states a notable hang glider flight was a 303 mile, 8 1/2 hour flight from new york to kansas. what was the average rate during that time?

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.