Three friends Pradeep, Suresh and Subodh workin the same office. During an extended weekend they
decided to go together on a family trip to Corbett National Park. They started from office in their own
car but at different time and travelled in the same direction at speeds of 50 km/hr, 60 km/hr and 75
km/hr respectively. Suresh and Subodh overtook Pradeep at the same point. If Suresh started 90
minutes after Pradeep, how many minutes after Pradeep did Subodh start from the office?

Consider the instance when Suresh just starts from the office. 
Distance that Pradeep has already covered in that time = 50*1.5 = 75 km 

Now, by concept of relative velocity, time taken by Suresh to just overtake Pradeep = 75/(60-50) = 7.5 hours
Hence, the point of overtaking on the route is = 60*7.5 = 450 km from the office 

Subodh also overtakes Pradeep at the same point. Pradeep covers this 450 km distance in 9 hours. 
Subodh can cover this 450 km in 6 hours. 
Hence, Subodh can leave 9 - 6 = 3 hours after Pradeep does.