Question

A car covers 4 successive stretches of 3 km each at speed of 10 kmph, 20 kmph, 30 kmph and 60 kmph respectively. The average speed of the car for the entire journey is:

• A. 15 kmph
• B. 20 kmph
• C. 25 kmph
• D. 35 kmph

Hint:

When the distance covered is the same for different speeds, the average speed is the Harmonic Mean of the individual speeds. For speeds \(s_1, s_2, ..., s_n\), the average speed is \( \frac{n}{\frac{1}{s_1} + \frac{1}{s_2} + ... + \frac{1}{s_n}} \). Here, it would be \( \frac{4}{\frac{1}{10} + \frac{1}{20} + \frac{1}{30} + \frac{1}{60}} = \frac{4}{12/60} = 20 \).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Average speed is calculated as the total distance traveled divided by the total time taken. It is not the simple arithmetic average of the speeds.
Step 2: Key Formula or Approach:
\[ \text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}} \] Where Total Time is the sum of the time taken for each stretch, and \( \text{Time} = \frac{\text{Distance}}{\text{Speed}} \).
Step 3: Detailed Explanation:
The car travels 4 stretches, each of 3 km.
Total Distance = \( 4 \times 3 \text{ km} = 12 \text{ km} \).
Now, let's calculate the time taken for each stretch:
- Time for 1st stretch (\(t_1\)) = \( \frac{3 \text{ km}}{10 \text{ kmph}} = 0.3 \text{ hours} \)
- Time for 2nd stretch (\(t_2\)) = \( \frac{3 \text{ km}}{20 \text{ kmph}} = 0.15 \text{ hours} \)
- Time for 3rd stretch (\(t_3\)) = \( \frac{3 \text{ km}}{30 \text{ kmph}} = 0.1 \text{ hours} \)
- Time for 4th stretch (\(t_4\)) = \( \frac{3 \text{ km}}{60 \text{ kmph}} = 0.05 \text{ hours} \)
Total Time = \( t_1 + t_2 + t_3 + t_4 \)
\[ \text{Total Time} = 0.3 + 0.15 + 0.1 + 0.05 = 0.6 \text{ hours} \] Now, calculate the average speed:
\[ \text{Average Speed} = \frac{12 \text{ km}}{0.6 \text{ hours}} = \frac{120}{6} = 20 \text{ kmph} \] Step 4: Final Answer:
The average speed of the car for the entire journey is 20 kmph.