Question : A car covers 3 successive 2 km stretches at speeds of 20 km/hr, 40 km hr, and 60 km/hr respectively. Its average speed is:
Option 1: 38.56 km/hr
Option 2: 42.12 km/hr
Option 3: 32.72 km/hr
Option 4: 40.50 km/hr
Correct Answer: 32.72 km/hr
Solution :
According to the question
First stretch at 20 km/hr:
⇒ Time t
1
= $\frac{2 km}{20km/hr}=\frac{1}{10}$ hr
Second stretch 40 km/hr:
⇒ Time t
2
= $\frac{2km}{40km/hr}=\frac{1}{20}$ hr
Third stretch 60km/hr:
⇒ Time t
3
= $\frac{2km}{60km/hr}=\frac{1}{30}$ hr
⇒ Total time = $\frac{1}{10} + \frac{1}{20} + \frac{1}{30} = \frac{11}{60}$ hrs
⇒ Total distance = 2 km + 2 km + 2 km = 6 km
⇒ Average speed = $\frac{\text{Total distance}}{\text{Total time}}$ = $\frac{6}{\frac{11}{60}}$ km/hr = 32.72 km/hr
Hence, the correct answer is 32.72km/hr.
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