To solve the problem, we follow these steps:

Step 1: Calculate the distance between A and B

Speed from A to B = 40 km/h, Time taken = 5 hours
Distance (d = \text{speed} \times \text{time} = 40 \times 5 = 200) km

Step 2: Total distance for the whole journey

Since it's a round trip, total distance = (2d = 2 \times 200 = 400) km

Step 3: Total time for the whole journey

Time from A to B = 5 hours, Time from B to A = 4 hours
Total time = (5 + 4 = 9) hours

Step 4: Calculate average speed

Average speed = (\frac{\text{Total distance}}{\text{Total time}} = \frac{400}{9} \approx 44.44) km/h

Answer: (\boxed{44.44}) (or (\frac{400}{9}) if exact fraction is preferred, but 44.44 is commonly accepted)

(\boxed{44.44})

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