Multiple choice

A man walks from his house to his dougther's school at a speed of $3$km/hr and return back at speed of $4$ km/hr. If he takes $21$ minutes for the double journey; find the distance between his house and the school.

  1. $0.7 km.$
  2. $0.2 km.$
  3. $0.6 km.$
  4. $0.9 km.$
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C Correct answer
Explanation

If the one-way distance is d, the total time is d/3 + d/4 = 21/60 hours. Thus, 7d/12 = 7/20, giving d = 0.6 km.

AI explanation

Using the formula for equal distances, the average speed for the entire journey is 2 multiplied by 3 multiplied by 4 divided by the sum of 3 and 4, giving 24/7 km/hr. The total time for the double journey is 21 minutes, which is 21 divided by 60 hours or 7/20 hours. Applying the formula Distance equals Speed multiplied by Time, the total distance is 24/7 multiplied by 7/20, which equals 1.2 km. Since this is a double journey, the one-way distance is 1.2 divided by 2, equalling 0.6 km.

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