Its Average Speed Over this Distance is :

Answer is (1): 20 km/hour

Explanation:-
A car covers four successive 7 km distances at the speeds of 10 km/hour, 20 km/hour, 30 km/hour and 60 km/hour respectively,
Then, the total time taken by four cars to covered the distance 7 km = 7/10 + 7/20 + 7/30 + 7/60
 = (7 x 6 + 7 x 3 + 7 x 2 + 7 x 1)/60
 = (42 + 21 + 14 + 7)/60
 = 84/60
 = 7/5 hours

The total distance covered by four cars = 7 + 7  + 7 + 7
 = 28 km

Then, the average speed of four cars = 28/(7/5)
 = 28 x 5/7
 = 4 x 5
 = 20 km/hr

Hence, the answer is (1) 20 km/hr.