Question : 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/hr during its entire journey?
Option 1: 60.0 km/hr
Option 2: 63.2 km/hr
Option 3: 62.0 km/hr
Option 4: 65.3 km/hr
Correct Answer: 63.2 km/hr
Solution :
In the first 10 minutes, the car travels at a speed of 21 m/s. [1 min = 60 sec then 10 min = 600 sec]
Distance travelled = 21 × 600 = 12600 m = 12.6 km
Total distance travelled = 12.6 + 9 + 10 = 31.6 km
Total time taken = (10 + 10 + 10) minutes = 30 min = $\frac{30}{60}$ = $\frac{1}{2}$ hour
$\therefore$ Average speed = $\frac{\text{Total distance}}{{\text{Total time}}}$ = $\frac{31.6}{\frac{1}{2}}$ = 31.6 × 2 = 63.2 km/hr
Hence, the correct answer is 63.2 km/hr.
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