A train running between two stations arrives at its destination
10 minutes late when it travels at 40 km/hr
and 16 minutes late when it travels at 30 km/hr.
Find the distance between the two stations.
Let = distance (in kilometers).
Let = time for a normal run (in hours).
We will use: .
At 40 kph, it is 10 minutes late.
At 30 kph, it is 16 minutes late.
Equate  and : .
Substitute into : .
But Captain Black has an elegant solution!