There are several information's already given in the question. Based on those information's the answer can be easily deduced.
Distance traveled by the bicycle rider = 50 km Time taken to travel the distance = 2.5 hours So In 2.5 hours the bicycle rider travels = 50 km Then In 1 hour the bicycle rider travels = 50/2.5 = 20 km Then the average speed of the cyclist is 20 km per hour.
