A) \[7\frac{2}{3}\] Hours
B) \[7\frac{3}{4}\] Hours
C) 8 Hours
D) \[\frac{7}{2}\] Hours
Correct Answer: D
Solution :
[d] Let speed in downstream \[=\left( x+y \right)\] Speed in upstream \[=\left( xy \right)\] \[\therefore\] \[\frac{d}{x+y}+\frac{d}{x-y}=\frac{21}{4}\] ? (1) As \[\frac{2d}{x-y}=7\] \[\therefore\] \[\frac{2d}{x+y}=\frac{21}{4}\times 2-7.\] \[=\frac{7}{2}hours\]
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