Help me with this: last night Ben and Alex went down to the park where they met John and Mary. They knocked John and Mary over and stole their bikes, riding off laughing to each other and exclaiming "wasn't that fun!" They rode the bikes up some really big hills, nearly a thousand feet of climbing. It was 9.4 miles to the top and when they got there Ben saw that they had averaged 12.4 miles an hour. Going back was nearly all downhill and when they got back to the park they had ridden 23.2 miles in 1 hour and 28 minutes at an overall average of 15.6 mph. They still agreed they had had fun but their legs felt tired. What was Ben and Alex's average speed from the top of the hill back to the park?