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Question

Abdul travels thrice the distance Catherine travels, which is also twice the distance that Binoy travels. Catherine's speed is $1/3$ of Abdul's speed, which is also $1/2$ of Binoy's speed. If they start at the same time then who reaches first?

The correct answer is
Binoy

Distance and Speed Analysis

To determine who reaches first, we need to compare the time taken by each person. The time taken is calculated using the formula: $t = \frac{d}{v}$ where $t$ is time, $d$ is distance, and $v$ is speed.

Relationship Definitions

Let the distances traveled be $d_A, d_C, d_B$ and speeds be $v_A, v_C, v_B$ for Abdul, Catherine, and Binoy, respectively.

  • Distance relationships:
    • $d_A = 3d_C$
    • $d_C = 2d_B$
  • Speed relationships:
    • $v_C = \frac{1}{3}v_A$
    • $v_A = \frac{1}{2}v_B$

Expressing Variables

Let's express all distances and speeds in terms of a common reference, say Binoy's distance ($d_B$) and Abdul's speed ($v_A$):

  • Distances:
    • Let $d_B = d$.
    • Then $d_C = 2d_B = 2d$.
    • And $d_A = 3d_C = 3(2d) = 6d$.
  • Speeds:
    • Let $v_A = v$.
    • Then $v_C = \frac{1}{3}v_A = \frac{1}{3}v$.
    • And $v_B = 2v_A = 2v$.

Calculating Travel Times

Now, calculate the time taken for each person using $t = d/v$:

  • Abdul's time: $t_A = \frac{d_A}{v_A} = \frac{6d}{v}$
  • Catherine's time: $t_C = \frac{d_C}{v_C} = \frac{2d}{(1/3)v} = \frac{6d}{v}$
  • Binoy's time: $t_B = \frac{d_B}{v_B} = \frac{d}{2v}$

Comparing Times and Conclusion

Comparing the calculated times:

  • $t_A = \frac{6d}{v}$
  • $t_C = \frac{6d}{v}$
  • $t_B = \frac{1}{2} \times \frac{d}{v}$

Since $\frac{1}{2} < 6$, the time $t_B$ is the smallest value among the three. The person who takes the least time reaches first.

Therefore, Binoy reaches first.

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Important Questions from Speed Time & Distance (Notes)

  1. Ajay walks at a speed of 4 km/hr. He doubles his speed after reaching exactly half way. He walks for 12 hours in all. What is the total distance travelled by him ?
  2. With a certain set of diameters, a tractor's front wheels make 2 revolutions (revs) for every 1 revolution of the rear wheels. The effect of changing wheel diameter(s) is shown in the following plot. 

     In this context, which of the following statements is CORRECT?

  3. A train travels from Karnal to Kurukshetra at a speed of $90$ km/hr and returns at a speed of $92$ km/hr. If it takes $182$ hours for total travel, then find the distance between Karnal to Kurukshetra. (In Km)
  4. 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:

  5. A cyclist travels at 18 km/hr for 2.5 hours. What distance does he cover?
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