Question 1157
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|||||||||||{---- are the trains at the moment they are at the same point.
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{----||||||||||| are the trains after passing each other.
Thus, the distance for the two trains to pass each other completely is 500m + 1000m = 1.5km 
(d)istance = (v)elocity x (t)ime
Thus, the time required for the trains to pass each other is t = d / v =
-> 1.5km/(100km/h + 80 km/h) = 1.5km/180km/hr
-> km cancels out -> convert to min
-> (1.5 x 60min/hr)/180/hr 
-> hr cancels out
-> 90min/180
-> 1min/2 = 30s to completely pass each other