The olympic record for the marathon is 2 h, 9 min, 21 s. the marathon distance is 26 mi, 385 yd. determine the average speed (in miles per hour) of this record.

The average speed of an object moving is its rate of change per time interval. It is equal to the total distance traveled divided by the total time it takes to travel that distance. We calculate the average speed of the olympic record as follows:

The units of the average speed should be miles per hour so we need to convert the distances in to miles and time into units of hours.
distance 1= 26 mi
distance 2 = 385 yd ( 1 mile / 1760 yd ) = 0.21875 mi
total distance = 26 + 0.21875 = 26.21875 mi

time 1 = 2 h
time 2 = 9 min ( 1 h / 60 min ) = 0.15 h
time 3 = 21 s ( 1 h / 3600 s ) = 0.0058 h
total time = 2 + 0.15 + 0.0058 = 2.1558 h

average speed = total distance / total time = 26.21875 mi / 2.1558 hr = 12.1620 mi / h

In this problem it decreased 20%