arithmetic - two tain 100 metres and 120 metres long are running in the same direction with speeds of 72 kmhr and 54 kmhr in how much time will the first train cross the second - skillgun

Relative speed of the train = (72 - 54) km/hr = 18 km/hr
= [18 * 5/18] m/sec = 5 m/sec.
Time taken by the trains to cross each other
= Time taken to cover (100 + 120) m at 5 m/sec = [220/5]sec = 44 sec.