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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. 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)
  3. A car covers a distance from Hayathnagar to LB nagar at the speed of 64 km/hr and covers the distance from LB nagar to Hayathnagar at the speed of 56 km/hr. What is the average speed (rounded off to nearest integer) of the car?
  4. A delivery van travels 156 km in 3.5 hours at a constant speed. What is its speed in km/hr (rounded off to the nearest whole number)?
  5. Priyanka covers 800 m distance in 6 minutes. Her speed in km/hr is:
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