UPSC 2017 CDS II Math Paper

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