It would take 4 hours for both trains to meet.
Speed is the ratio of distance travelled to time taken. It is given by:
Speed = distance / time
Let t represent the time at which both trains meet, and d represent the distance covered by train A, hence:
For train A:
100 = d / t
t = d/100
For train B:
200 = (1200 - d) / t
t = (1200 - d) / 200
d/100 = (1200 - d)/200
2d = 1200 - d
3d = 1200
d = 400 km
t = 400 / 100 = 4 hours
It would take 4 hours for both trains to meet.
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