A train running at 4km/h leaves a railway station 21 hours later than another train, and meets it in 7 hours. What is the speed of the other train?
Let the speed of the train be v km/h. Distance travelled by this train in (7 + 21) hours = 28v km. Equating this to the distance travelled by the second train we get 28v = 4 × 7, which gives v = 1 km/h.