A person travels at the speeds of 36 km/h, 30 km/h and 45 km/h on the surface of an equilateral triangular ground. Find his average speed?

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RPF SI (2018) Official Paper (Held On: 24 Dec, 2018 Shift 1)
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  1. 45 km/h
  2. 36 km/h
  3. 20 km/h
  4. 30 km/h

Answer (Detailed Solution Below)

Option 2 : 36 km/h
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Detailed Solution

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Given:

Speed on the first side = 36 km/h

Speed on the second side = 30 km/h

Speed on the third side = 45 km/h

Formula Used:

Average speed = Total distance travelled / Total Time

Calculation:

Let each side of the equilateral triangle be of length d km.

Time taken on the first side = d / 36

Time taken on the second side = d / 30

Time taken on the third side = d / 45

Total Distance = d + d + d = 3d

Total Time = (d / 36) + (d / 30) + (d / 45)

Total Time = d(1/36 + 1/30 + 1/45)

Total Time = d(5/180 + 6/180 + 4/180)

Total Time = d(15/180)

Total Time = d(1/12)

Average Speed = (3d) / (d/12)

Average Speed = (3d × 12) / d

Average Speed = 36 km/h

The average speed is 36 km/h.

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