A train is running at the speed of 25 km/hr from Delhi to Jaipur at 2 pm. After one hour another train is running from Jaipur to Delhi at the speed of 15 km/hr. If they are running on parallel tracks and the distance between A and B is 305 km, then at what time will they start to cross each other ?

10 pm

Train 2 starts after 1 hour of train 1 i.e. at 3 PM  with speed 15 km/hr

From 2 PM to 3 PM , only train 1 travels . so distance traveled by train 1 in 1 hour with 25km/hr speed is ,

Distance = Speed × Time

Distance = 25 × 1 = 25 km

When train 2 started , distance left to travel  = 305 - 25 = 280 km

Relative speed when both trains moves in opposite direction is  = ( 15 + 25 ) = 40 km/hr 

Time taken to cross each other  = \(\frac{280}{40}\)  = 7 hours 

As time count of both trains together start at 3 PM . so they cross each other at 3 PM + 7 hours 

= 10 PM