Step-by-step explanation:
Let the time of catch up be t hours after the start of second train start.
Time traveled by first train = t + (6-3) = t + 3
Speed of first train = 30 mph
Speed of second train = 60 mph
Distance traveled by first train = Distance traveled by second train
30 x ( t + 3 ) = 60 t
30t + 90 = 60t
30 t = 30
t = 3 hours.
After the start of second train after 3 hours second train overtakes first train.