there are 4 people who has to cross a stretch of 300 km . they normally run at a speed of 10 kmph . one of them has a bike that travels at 50 kmph . the bike first takes one person alone and crosses the stretch while the other two keep running . then he comes back without wasting time and picks up another person from the way , drives him across the stretch , and does the same for the last person . how long does this whole process take ?