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Question

Four villages A. B. C and D are connected in that order by a circular road. A car traveling with a uniform speed covers the distance between A and B in 43 minutes. B and C in 23 minutes. C and D in 19 minutes and D and A in 47 minutes. Which of the following will be closest to the time (in minutes) taken to travel from A to C with the same speed along a straight road?

The correct answer is

42

The question provides information about the travel time between four villages A, B, C, and D connected by a circular road, assuming a uniform speed. We need to find the approximate time taken to travel from A to C along a straight road with the same speed.

Villages and Circular Road Travel Time

The times taken to travel between consecutive villages along the circular road are:

  • A to B: 43 minutes
  • B to C: 23 minutes
  • C to D: 19 minutes
  • D to A: 47 minutes

Since the car travels at a uniform speed, the distance between any two points is directly proportional to the time taken to travel between them. Let the speed be $v$.

Travel Time Along the Circular Road from A to C

There are two paths along the circular road from A to C:

  • Path 1: A → B → C. The time taken is the sum of times from A to B and B to C: $43 + 23 = 66$ minutes.
  • Path 2: A → D → C. The time taken is the sum of times from A to D and D to C: $47 + 19 = 66$ minutes.

Both paths along the circular road from A to C take 66 minutes. This equality suggests that A and C might be positioned such that the circular path distance between them is the same in both directions around the loop. A common geometric interpretation for this scenario on a circular path is that A and C are diametrically opposite points on the conceptual circle.

Relating Straight Road Distance to Circular Road Distance

The question asks for the time taken along a straight road from A to C. This straight road represents the direct distance, which would be a chord connecting points A and C. If A and C are diametrically opposite points on a circle, the straight-line distance is the diameter, and the circular distance along either path (A-B-C or A-D-C) is half the circumference.

Let $R$ be the radius of the circle. The straight distance (diameter) between A and C is $2R$. The circular distance (half circumference) between A and C is $\pi R$.

The ratio of the straight distance to the circular distance is:

$$ \frac{\text{Straight Distance (A to C)}}{\text{Circular Distance (A to C)}} = \frac{2R}{\pi R} = \frac{2}{\pi} $$

Since speed is uniform, the ratio of times taken is equal to the ratio of distances:

$$ \frac{\text{Time (Straight A to C)}}{\text{Time (Circular A to C)}} = \frac{\text{Straight Distance (A to C)}}{\text{Circular Distance (A to C)}} = \frac{2}{\pi} $$

Calculating Travel Time Along the Straight Road

We know the time taken along the circular road from A to C is 66 minutes. Using the ratio derived above:

$$ \text{Time (Straight A to C)} = \text{Time (Circular A to C)} \times \frac{2}{\pi} $$

$$ \text{Time (Straight A to C)} = 66 \times \frac{2}{\pi} $$

To find the closest option, we can use a common approximation for $\pi$, such as $\pi \approx \frac{22}{7}$:

$$ \text{Time (Straight A to C)} \approx 66 \times \frac{2}{\frac{22}{7}} = 66 \times \left(2 \times \frac{7}{22}\right) = 66 \times \frac{14}{22} $$

Simplify the expression:

$$ \text{Time (Straight A to C)} \approx 66 \times \frac{7}{11} = \left(\frac{66}{11}\right) \times 7 = 6 \times 7 = 42 \text{ minutes} $$

Using a more precise value for $\pi$, like 3.14159:

$$ \text{Time (Straight A to C)} \approx 66 \times \frac{2}{3.14159} \approx 66 \times 0.6366 \approx 41.996 \text{ minutes} $$

Both calculations yield a value very close to 42 minutes.

Therefore, the time taken to travel from A to C with the same speed along a straight road is closest to 42 minutes.

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Important Questions from Average Speed

  1. A car runs first 275 km at an average speed of 50 km/h and the next 315 km at an average speed of 70 km/h. What is the average speed ( in km/h) for the entire journey?

  2. Akhil rides first 12 km at a speed of 16 km/h and further 6 km at a speed of 20 km/h. Find his average speed (in km/h).

  3. Shyam drives his car 30 km at a speed of 45 km/h and, for the next 1 h 20 m, he drives it at a speed of 51 km/h. Find his average speed (in km/h) for the entire journey.

  4. X and Y travel a distance of 90 km each such that the speed of Y is greater than that of X. The sum of their speeds is 100 km/h and the total time taken by both is 3 hours 45 minutes. The ratio of the speed of X to that of Y is:

  5. If a man travels at \(\frac{1}{x}\) km/h on a journey and returns at  \(\rm \frac{1}{x^2}\) km/h, then his average speed for the journey is:

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