The 2 minute and the 1 minute person go across (time = 2 min). Then the 1 minute person comes back (cumulative time = 3 min). Then the 5 and 10 minute people go across (cumulative time = 13 min). Then the two minute person goes back (cumulative time = 15 minutes). Finally the 2 and 1 minute people go back across (cumulative time =17 minutes).At step two, the 2 minute person can go back and then the 1 minute person goes back at step four - but the result is the same (17 minutes).