BOAT AND STREAMS –01

Question: A man can row at a speed of 5 km/hr in still water to a certain upstream point and back to the starting point in a river which flows at 2 km/hr. Find his average speed for total journey. [एक व्यक्ति, जो शांत जल में 5 किमी/घंटे की चाल से चल सकता है; धारा के विरुद्ध एक निश्चित दूरी तक जाता है और प्रारंभ बिंदु तक वापस पहुँचता है. यदि धारा की चाल 2 किमी/घंटे हो, तो व्यक्ति की औसत चाल बताएं]. [Ans. (b) 4.2 km/hr]                                        

(a) 9 km/hr  (b) 4.2 km/hr (c) 8 km/hr  (d) 6 km/hr  (e) None of these

Solution:

Detailed Explanation: Given that the speed of the man in still water is 5 km/h and the speed of the current is 2 km/h. So, the speed of the man along the current, X = 5+2 = 7 km/h; and the speed of the man against the current, Y = 5-2 = 3 km/h.

Average speed of the man = (2XY)/(X+Y) km/h

= (2×7×3)/7+3 km/h = 42/10 km/h = 4.2 km/h. Ans.