Q. Tim paddles his kayak upstream from his home to his friend's home, taking 2.5 hours for the journey. He returns that afternoon in 1.5 hours. If the river is flowing at 2.5km/h, find his paddling speed in still water.

i check the answers and its 10km/h but i have no idea how to work it out.

thanks

edit: dw i worked it out: