arithmetic - two trains 140 m and 160 m long run at the speed of 60 kmhr and 40 kmhr respectively in opposite directions on parallel tracks the time in seconds which they take to cross each other is - skillgun

Relative speed = (60 + 40) km/hr = 100 x 5/18 m/sec = 250/9 m/sec.
Distance covered in crossing each other = (140 + 160) m = 300 m.
Required time = 300 x 9/250 sec = 54/5 sec = 10.8 sec.