Whats the first step*

-4 = n + 5

Trip uphill to the store:
distance = speed * time
               = (15mph)*(12minutes/60 minutes per hour)
               = (15mph)*(0.2 hours)
               = 3 miles

trip downhill back home is along the same path
so the trip downhill was also equal to 3 miles.

So the the entire trip was a distance of 6 miles
3 miles uphill and 3 miles downhill.

So the average speed = (15mph uphill + 30mph downhill)/2
                                    = 45 mph/2
                                    = 22.5 mpg

So the average speed of the entire trip uphill and downhill is 22.5 mph