A goods train 158 metres long and travelling at the speed of 32 km per hr. leaves Delhi at 6 am. Another mail train 130 metres long and travelling at the average speed of80 km per hr. leaves Delhi at 12 noon and follows the goods train. At what time will the mail train completely cross the goods train?

(1)4hours (2)4 hours 21.6 sec. (3)5 hours 21.6 sec. (4) None of these

Solution:

The goods train leaves Delhi at 6 am and mail train at 12 noon, hence after 6 hours

The distance covered by the goods train in 6 hours at 32 km per hr. 

= 32*6= 192 kms

The relative velocity of mail train with respect to goods train 

= 80-32 = 48 km per hr.

To completely cross the goods train, the mail train will have to cover a distance

= 192 km + 158m + 130m

= 192km + 0.158 km + 0.130 km

= 192.288 km more

Since. the mail traingoes 48kms more in 1 hour.

:. The mail train goes 192.288 kms more in

=192288/1000*1/48=2003/500

= 4 hours 21.6 sec.