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Bunuel
A man starts rowing from Point A in the direction of the water flow at a constant speed of 10 kilometers per hour. After rowing for 10 kilometers, he remembers that he forgot an important object at Point A and turns back to get it. After getting the object, he starts moving again in the direction of water flow, but due to an onset of exhaustion, he now rows at a constant speed of 8 kilometers per hour. After rowing for 26 kilometers, he reaches Point D, where he rests for 1 hour, and then resumes his journey in the same direction, rowing at a constant speed of 9 kilometers per hour, till he reaches Point B, which is 14 kilometers downstream from Point D. If the speed of water flow is 5 kilometers per hour, what is the average speed of the man for the entire journey?

(A) 9 kilometers per hour

(B) 130/17 kilometers per hour

(C) 120/17 kilometers per hour

(D) 20/3 kilometers per hour

(E) 6 kilometers per hour
Solution:

  • A to Realization Point (downstream)
    • Speed \(10+5=15\) kmph
    • Distance \(=10\) km
    • Time \(=\frac{Distance}{Time}=\frac{10}{15}=\frac{2}{3}\) hr

  • Realization Point to A (upstream)
    • Speed \(10-5=5\) kmph
    • Distance \(=10\) km
    • Time \(=\frac{Distance}{Time}=\frac{10}{5}=2\) hr

  • A to D (downstream)
    • Speed \(8+5=13\) kmph
    • Distance \(=26\) km
    • Time \(=\frac{Distance}{Time}=\frac{26}{13}=2\) hr

  • Rest time \(=2\) hr

  • D to B (downstream)
    • Speed \(9+5=14\) kmph
    • Distance \(=14\) km
    • Time \(=\frac{Distance}{Time}=\frac{14}{14}=1\) hr

  • Total distance \(=10+10+26+14=60\) km
  • Total time \(=\frac{2}{3}+2+2+1+1=6.67\) hrs
  • Average speed \(=\frac{Total_Distance}{Total_Time}=\frac{60}{6.67}=9\) (almost)

Hence the right answer is Option A

Hi small issue here:

The rest time is 1 not 2 .

Answer will come exactly 9.
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Bunuel
A man starts rowing from Point A in the direction of the water flow at a constant speed of 10 kilometers per hour. After rowing for 10 kilometers, he remembers that he forgot an important object at Point A and turns back to get it. After getting the object, he starts moving again in the direction of water flow, but due to an onset of exhaustion, he now rows at a constant speed of 8 kilometers per hour. After rowing for 26 kilometers, he reaches Point D, where he rests for 1 hour, and then resumes his journey in the same direction, rowing at a constant speed of 9 kilometers per hour, till he reaches Point B, which is 14 kilometers downstream from Point D. If the speed of water flow is 5 kilometers per hour, what is the average speed of the man for the entire journey?

(A) 9 kilometers per hour

(B) 130/17 kilometers per hour

(C) 120/17 kilometers per hour

(D) 20/3 kilometers per hour

(E) 6 kilometers per hour
Solution:

  • A to Realization Point (downstream)
    • Speed \(10+5=15\) kmph
    • Distance \(=10\) km
    • Time \(=\frac{Distance}{Time}=\frac{10}{15}=\frac{2}{3}\) hr

  • Realization Point to A (upstream)
    • Speed \(10-5=5\) kmph
    • Distance \(=10\) km
    • Time \(=\frac{Distance}{Time}=\frac{10}{5}=2\) hr

  • A to D (downstream)
    • Speed \(8+5=13\) kmph
    • Distance \(=26\) km
    • Time \(=\frac{Distance}{Time}=\frac{26}{13}=2\) hr

  • Rest time \(=2\) hr

  • D to B (downstream)
    • Speed \(9+5=14\) kmph
    • Distance \(=14\) km
    • Time \(=\frac{Distance}{Time}=\frac{14}{14}=1\) hr

  • Total distance \(=10+10+26+14=60\) km
  • Total time \(=\frac{2}{3}+2+2+1+1=6.67\) hrs
  • Average speed \(=\frac{Total_Distance}{Total_Time}=\frac{60}{6.67}=9\) (almost)

Hence the right answer is Option A

Hi small issue here:

The rest time is 1 not 2 .

Answer will come exactly 9.

Hey, thank you for pointing out the typo mistake there.
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A man starts rowing from Point A in the direction of the water flow at a constant speed of 10 kilometers per hour. After rowing for 10 kilometers, he remembers that he forgot an important object at Point A and turns back to get it. After getting the object, he starts moving again in the direction of water flow, but due to an onset of exhaustion, he now rows at a constant speed of 8 kilometers per hour. After rowing for 26 kilometers, he reaches Point D, where he rests for 1 hour, and then resumes his journey in the same direction, rowing at a constant speed of 9 kilometers per hour, till he reaches Point B, which is 14 kilometers downstream from Point D.

If the speed of water flow is 5 kilometers per hour, what is the average speed of the man for the entire journey?

Distance travelled and times taken: -

First 10 km from point A
Distance travelled = 10 km
Time taken = 10/(10+5) = 10/15 = 2/3 hours

Turn back 10 km to point A
Distance travelled = 10 km
Time taken = 10/(10-5) = 10/5 = 2 hours

Rowing 26 km to point D
Distance travelled = 26 km
Time taken = 26/(8+5) = 26/13 = 2 hours

Rest 1 hour

Rowing 14 km to point B
Distance travelled = 14 km
Time taken = 14/(9+5) = 1 hours

Total distance travelled = 2*10 + 26 + 14 = 60 km
Time taken = 2/3 + 2 + 2 + 1 + 1 = 6 2/3 = 20/3 hours
Average speed of the man for the entire journey = 60/(20/3) = 9 kmph

IMO A
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If he is resting in the rowing boat, then he would have moved 5Km during that 1 hour. Why we are not considering this distance?
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