Multiple choice

The speed of a boat is thrice that of the speed of the current. The total time taken by a boatman to reach its starting point from a point 72 km farther to the departure point is 10.8 hours. What is the speed of the boat in downstream? एक नाव की चाल धारा की चाल का तीन गुना है | नाविक द्वारा 72 किमी. दूर स्थित किसी बिंदु से अपने मूल बिंदु पर वापस आने में 10.8 घंटे लगते हैं | नाव की चाल अनुप्रवाह में क्या है ?

  1. 15 km/hr. /किमी. /घंटा

  2. 20 km/hr. /किमी. /घंटा

  3. 5 km/hr. /किमी. /घंटा

  4. 10 km/hr. /किमी. /घंटा
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B Correct answer
Explanation

Boat speed = 3c where c is current speed. Downstream speed = 3c + c = 4c. Upstream speed = 3c - c = 2c. Total distance = 72 + 72 = 144 km. Total time = 10.8 hours. Time = distance/speed: 72/4c + 72/2c = 18/c + 36/c = 54/c = 10.8. Therefore c = 54/10.8 = 5 km/hr. Downstream speed = 4c = 20 km/hr.

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