Question 27
Four girls need to cross a dark narrow tunnel and they have only a torch among them. Girl A can cross the tunnel in 1 minute. Girl B can cross the tunnel in 2 minutes. Girl C can cross the tunnel in 5 minutes. Girl D can cross the tunnel in 10 minutes. Given that they need a torch to cross at all times and that the tunnel can only allow two girls to go through at any given time, find the least possible time for the four girls to get across the tunnel. Note: the time taken by the slower girl is taken to be the time of each crossing. Enter the time in minutes.