A group of 5th graders went on a trip. Some of them had bikes and some did not. The ones without bikes left at 9:00 AM, walking at an average speed of 5 kilometers per hour. The ones with bikes can bi

A group of 5th graders went on a trip. Some of them had bikes and some did not. The ones without bikes left at 9:00 AM, walking at an average speed of 5 kilometers per hour. The ones with bikes can bike with the average speed of 12 kilometers per hour. They plan to leave later, but they need to catch up with the rest of the group before they all stop for the lunch at noon. When should they start biking?