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Question
A boat sails m miles upstream at the rate of r miles per hour. If the rate of the stream is s miles per hour, how long will it take the boat to return to its starting point?
m/r+2s
m/r+s
m+r/s
m+3r/s
None of these
ANSWER : 1
Descrption
<p>Let the speed of the boat in still water be “x” miles/hr.</p> <p>The speed of the stream is given as “s” miles/hr</p> <p>Therefore,</p> <p>Speed of the boat upstream = (x – s) miles/hr ….. (i)</p> <p> </p> <p>Speed of the boat downstream = (x + s) miles/hr ……. (ii)</p> <p>The distance covered by the boat while sailing upstream is given as “m” miles</p> <p>Speed of the boat upstream is given as “r” miles/hr …. (iii)</p> <p>From (i) & (iii), we get</p> <p>x - s = r</p> <p>⇒ x = r + s ….. (iv)</p> <p>Substituting the value of x in eq. (iii), we get</p> <p>Speed of the boat downstream = (r + s + s) miles/hr = (r + 2s) miles/hr ….. (v)</p> <p>Time = distance / speed</p> <p> = [m / (r + 2s)] hr</p> <p> </p>
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