My take on the first riddle.
If I take the prisoner at the back to be 1 and the prisoner in the front to be 30. Prisoner 1 being the one with nobody looking at his back.
Prisoner 1 would say prisoner 2's colour. This ensures Prisoner 2's survival. since Prisoner 2 is saved and couldn't save...
This problem took me a while but once you figure it out, you'll kick yourself in the butt. Naturally, you think 19 minutes is the time you need for them to all go across but it's actually only 16.
What I thought was that if 10 and 5 go separate, it's 15 minutes which is obviously too much. So...