Correct Answer - Option 3 : 6 km/h
Concept:
-
Speed: The distance travelled by the body in unit time.
- \(Speed = \frac{{Distance}}{{Time}}\)
- Also, \(Time= \frac{{Distance}}{{Speed}}\)
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Average Speed: Since the body does not always have the same speed, So we use the concept of average speed. It is the ratio of total distance covered to total time taken by the body.
- \(Average\;Speed = \frac{{Total\;Distance}}{{Total\;Time}}\)
Calculation:
Total Distance = 4 km
Distance travelled in first 10 min at speed of 12 km/hr = Speed × time = 12 × (10/60) = 2 km
Time taken to cover the distance of last 2 km = Distance/speed = 2/8 = 1/4 = 0.25 hr = 15 min
Total time = 10 min + 15 min (Chat with friend) + 15 min = 40 min = 2/3 hr
We know that,
\(Average\;Speed = \frac{{Total\;Distance}}{{Total\;Time}}\)
\(⇒ Average\;Speed = \frac{{4\;km}}{{2/3\;hr}} = 6 \;km/hr\)
⇒ Average Speed = 6 km/hr
