Answer:
Average speed =  miles per hour
 miles per hour
Step-by-step explanation:
Formula to calculate the average speed of an object is,
Average speed = 
If I travel with the speed of x mph for t hours,
Total distance covered = Speed × Time 
                                        =  
 
                                        =  miles
 miles
Similarly, if I travel with the speed y mph for t hours,
Total distance covered =  miles
 miles
Total distance covered by me =  miles
 miles
Total time taken to cover this distance = 
                                                                  =  hours
 hours
Therefore, expression for the average speed will be,
Average speed = 
                           = 
                           =  miles per hour
 miles per hour