With an average speed of 50 km/hr, a train reaches its destination on time. If it goes at an average speed of 40 km/hr, it is late by 24 minutes. The total distance travelled is:

The total distance is 80 km.

The delay of 24 min = 24/60 = 0.4 hour is the extra time at the slower speed.

For distance D, D/40 - D/50 = 0.4.

D(1/40 - 1/50) = D/200 = 0.4.

D = 0.4 x 200 = 80 km.