Distance between 91 km stones = 90 km Time taken by the train to cross 91 km stones =
90
60
=
3
2
hr=1hr 30 min Time taken to cover the first km stone = 12 sec Therefore, time taken by the train to cross 91–km stones completely = 1 hr 30 min + 12 sec = 1 hr 30 min 12 sec