Explanation

To find the average speed for the entire journey, let's assume the total distance traveled is 3x (since he travels equal distances at each speed).

Let's denote the speeds as:

S1 = 40 km/h

S2 = 30 km/h

S3 = 15 km/h

Time taken for each segment:

T1 = x / 40

T2 = x / 30

T3 = x / 15

Total time = T1 + T2 + T3

= x / 40 + x / 30 + x / 15

To find the common denominator (120):

Total time = 3x / 120 + 4x / 120 + 8x / 120

= 15x / 120

Total distance = 3x

Average speed = Total distance / Total time

= 3x / (15x / 120)

= 3 * 120 / 15

= 24