Five friends Alice, Bob, Carole, Dylan and Emma are heading to a common destination 100 unit distance away. They start together. Grandma Alice walks at a speed of 1. Bob and Carole walk at speeds 4 and 5, respectively. Dylan uses a skateboard and is able to travel at speed 7. The roller skating Emma travels at speed 9. It's kind of late, so they want to arrive at the destination as quickly as possible. Just as they're worrying about the lack of transportations, two friendly motorcyclists pass by and are willing to help out. The motorcycles are able to travel at speeds 10 and 15 respectively. One motorcyclist can carry only one passenger at a time, but of course, the passenger can get off any time so another person can get on board when needed.
We assume getting on and off the motorcycles take no time at all, and ignore the deceleration when dropping off and picking up a person. The motorcyclists don't mind going back and forth many times. The road is wide so there's no problem for one vehicle to overtake or pass by another vehicle or person.
What's your plan to minimize the amount of time it takes for all of the five friends to arrive at the destination?