Two trains each 500 m long are moving on parallel tracks towards each other at speeds 10 m/s and 15 m/s respectively. In how much time will they pass each other?

Length of each train = 500 m

Effective distance which needs to be covered by each train to cross over = 500 + 500 = 1000 m

velocity of first train = 10 m/s

velocity of second train = 15 m/s

net relative velocity = 15 - (-10) (as the trains are travelling in opposite directions)

= 25 m/s

time taken to travel 1000 with velocity 25 m/s = 1000 / 25 = 40 seconds