A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance with a speed of 20 km/hr and the last one-third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is :

Question: A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance with a speed of 20 km/hr and the last one-third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is :

Solution:
Let, the distance travelled by a car be x km
First x/3 km distance cover at a speed of 10 km/hr
Second x/3 km distance cover at speed = 20 km/hr
Third x/3 km distance cover at speed = 60 km/hr

According to the question,
Total time = x/(3 × 10) + x/(3 × 20) + x/(3 × 60)
= x/30 + x/60 + x/180
= (6x + 3x + x)/180
= 10x/180
= x/18

Average speed = (x × 18)/x km/hr = 18 km/hr