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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. A and B have to travel from place P to place Q following the same route in their respective cars. A drives at $60$ kmph while B drives at $80$ kmph. Find the time taken by B to reach place Q if A takes $12$ hrs.

  2. On a straight road, a bus is $60$ km ahead of a car running in the same direction. After $3$ hours, the car is $90$ km ahead of the bus. If the speed of the bus is $45$ km/h, then what is the speed of the car (in km/h)?

  3. A train running at the speed of $90$ kmph crosses a $250$ m long platform in $26$ seconds. What is the length of the train (in m)?

  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 car travels a total distance L. It travels half the distance with speed $v_1$ and the other half with speed $v_2$. The average speed of the car is :
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