In order to watch the South China 20 10 World Cup, eight fans rushed to the airport in two cars, one of which broke down at a distance of 15 km. At this time, it is stipulated that the time to arrive at the airport is 42 minutes, but the only available means of transportation is a car, including the driver, limited to 5 people. The car was sent to the airport in two batches, and the average speed was