A car travels with a speed of 21 m/sec in the first 10 minutes, 9.0 km in the next 10 minutes and 10 km in the last 10 minutes of its journey. What is the average speed of the car in km/h during its entire journey?

63.2 km/h

Formula used 

Distance = Speed × Time

In the first 10 minutes car travels with a speed of 21m/s             [1 min = 60 sec then 10 min = 600 sec]

Distance traveled = 21 × 600 = 12600 m

= 12.6 km

Total distance traveled = 12.6 + 9 + 10 = 31.6 km

Total time taken = (10 + 10 + 10) minutes = 30 min

= \(\frac{30}{60}\) = \(\frac{1}{2}\) h

Average speed = \(\frac{31.6}{0.5}\) = 31.6 ×  2 = 63.2 km/h

Hence, average speed is 63.2 km/h .