A man covered a certain distance at an average speed of 70 km/hr and comes back at the speed of 30 km/hr. Find the average speed (in km/hr) of the man during the whole journey?

42 km/hr

Let speed of man while going = A 

speed of man while coming back = B 

Average speed = \(\frac{2 ×A × B  }{A + B}\)

= \(\frac{2 ×70 × 30  }{70 + 30}\) = \(\frac{2 × 2100  }{100}\) = 42 km/hr