A man goes from A to B At the speed of 5 kmph and returns from B to A at 4 kmph. His average speed during the whole journey is
Let the distance between A and B be d
Time taken for travel from A to B is d/5
Time taken for travel from B to A is d/4
Average speed = total distance/ total time = $$ \frac{2d}{d/4 + d/5}$$
= $$ \frac{40}{9} $$
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