A man covers a certain distance by car driving at 30 km/hr and he returns back to the starting point riding on a scooter at 10 km/hr. Find his average speed for the whole journey. Choose the correct option:
A) 8 Km/hr B) 17 Km/hr C) 15 Km/hr D) 12 km/hr E) None of these
Explanation:   = (2 x a x b)/(a + b)   = (2 x 30 x 10)/(30 + 10)   = (2 x 30 x 10)/40   = 15 km/hr   Answer: Option 15 Km/hr
<result>C</result>
A man covers a certain distance by car driving at 30 km/hr and he returns back to the starting point riding on a scooter at 10 km/hr. Find his average speed for the whole journey.