A train is running at the speed of 40 km/hr from Delhi to Jaipur at 5 pm. After one hour another train is running from Jaipur to Delhi at the speed of 30 km/hr. If they are running on parallel tracks and the distance between A and B is 390 km, then at what time will they start to cross each other ?

11 pm

Train 2 starts after 1 hour of train 1 i.e. at 6 PM  with speed 30 km/hr

From 5 PM to 6 PM , only train 1 travels . so distance traveled by train 1 in 1 hour with 40km/hr speed is ,

Distance = Speed × Time

Distance = 40 × 1 = 40 km

When train 2 started , distance left to travel  = 390 - 40 = 350 km

Relative speed when both trains moves in opposite direction is  = ( 30 + 40 ) = 70 km/hr 

Time taken to cross each other  = \(\frac{350}{70}\)  = 5 hours 

As time count of both trains together start at 6 PM . so they cross each other at 6 PM + 5 hours 

= 11 PM