Given data:

Half the distance at a speed of 25 km/hour,

40% of the remaining distance at a speed of 8 km/hour,

For the remaining distance at a speed of 18 km/hour;

Concept:

Use the formula for average speed = \(\rm \frac{total\ distance}{total \ time}\)

Solution:

Let the total distance be 2D.

⇒ Time for half the distance = \(\rm \frac{D}{25}\)

⇒ Time for 40% of the remaining distance = \(\rm \frac{0.4\ D}{8}\)

⇒ Time for the remaining distance = \(\rm \frac{0.6\ D}{18}\)

⇒ Total time = \(\rm \frac{D}{25}+\frac{0.4D}{8}+\frac{0.6D}{18}=\frac{37D}{300}\)

⇒ Average speed = \(\rm \frac{2D}{37D/300}=\frac{600}{37}\) km/hour

Thus, the required average speed  \(\frac{600}{37}\) km/hour is.