1. Stanley drew level again with his friend as they cycled along the road at a steady 12 miles per hour. "That's another blue bus, like the one that overtook us exactly 20 minutes ago," he shouted. "Yes, the next one of the same run," replied Dennis, glancing at his watch. "And right on time. There's one every fourteen minutes this time of day." What was the average speed of the buses?

28 mph

52 mph

40 mph