Little Red Riding Hood #1

Little Red Riding Hood decided to go on a walk in the forest. She walks with an amazing precision of exactly 10 km/h . She has been walking for some time when she meets her grandma, who also walks with an amazing precision of 5 km/h . They stop and have a chat for some time. As they are getting tired, a huntsman drives up to them in a carriage driving at an amazing precision of 20 km/h and offers them a ride. As their houses are in opposite directions , Little Red Riding Hood decides to let grandma ride the carriage while she walks back home. The huntsman and grandma drive on for half the trip to her house when they are stopped by a huge log blocking their way. The grandma thanks the huntsman and says that she can manage on her way home. She also tells the huntsman to drive back to Little Red Riding Hood so that he can take her home. On the other hand, Little Red Riding Hood has been walking for exactly one hour and has sit down to rest. The carriage catches up with her and the huntsman offers her a ride home. She accepts, and both Little Red Riding Hood and her grandma get back home. Over this journey, the time her grandma has used walking back home and the time when Little Red Riding Hood has been sitting in the carriage are the same. All time used in different events, except the time used for travelling, are negligible. What is the shortest distance between Little Red Riding Hood's house and her grandma's house? (You only need to type the number.)