Atrain leave a station traveling north at 50km/hr. 3 hours later a second train leaves on parrallel tracks travels north at 90km/hr. how far from the station will they meet?

-- The first train leaves the station,going 50km/hr. 

-- 3 hours later, when the 2nd train starts out, the first train
has covered (3 x50) = 150 km.

-- When they're both moving, the 2nd train gains (90 - 50) = 40km
on the first train each hour.

-- The second train gains the 150km that he was behind, and overtakes
the first train, in (150 / 40) = 3.75 hours .

-- In that 3.75 hours, the second train has covered (3.75 x 90) = 337.5km ,
and that's the distance from the station to the place where they 'meet'.