A train is running at the speed of 30 km/hr from Delhi to Jaipur at 4 pm. After one hour another train is running from Jaipur to Delhi at the speed of 20 km/hr. If they are running on parallel tracks and the distance between A and B is 230 km, then at what time will they start to cross each other ?

09.00 pm

Train 2 starts after 1 hour of train 1 i.e. at 5 PM  with speed 20 km/hr

From 4 PM to 5 PM , only train 1 travels . so distance traveled by train 1 in 1 hour with 30 km/hr speed is ,

Distance = Speed × Time

Distance = 30 × 1 = 30 km

When train 2 started , distance left to travel  = 230 - 30 = 200 km

Relative speed when both trains moves in opposite direction is  = ( 30 + 20 ) = 50 km/hr 

Time taken to cross each other  = \(\frac{200}{50}\)  = 4 hours 

As time count of both trains together start at 5 PM . so they cross each other at 5 PM + 4 hours 

= 9 PM