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CBSE Class 10 Answered

The question is :-A boat on a river travels downstream between two points 20 km apart in one hour.The return trip against the current taken two and half hours.What is the speed of the boat?

(A)8 km/hr  (B)10 km/hr  (C)14 km/hr  (D)16km/hr

Asked by Udit Lilhare | 09 Apr, 2014, 10:16: AM
answered-by-expert Expert Answer
Important Formulae:

  1. Downstream/Upstream:

    In water, the direction along the stream is called downstream. And, the direction against the stream is called upstream.

  2. If the speed of a boat in still water is u km/hr and the speed of the stream is vkm/hr, then:

    Speed downstream = (u + v) km/hr.

    Speed upstream = (u - v) km/hr.

  3. If the speed downstream is a km/hr and the speed upstream is b km/hr, then:

    Speed in still water = 1 (a + b) km/hr.
    2
    Rate of stream = 1 (a - b) km/hr.
    2
Solution:         Rate downstream =open parentheses 20 over 1 close parentheses = 20
Rate upstream =fraction numerator 20 over denominator begin display style 5 over 2 end style end fraction=8
 Speed in still water =1 half(20+8) kmph = 14 km/hr
 
Answered by | 09 Apr, 2014, 11:35: AM

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