A) 3 km/h
B) 4 km/h
C) 2 km/h
D) 5 km/h
Correct Answer: A
Solution :
| [a] Let rate of flow of river be u km/h. Then,\[\frac{91}{10+u}+\frac{91}{10-u}=20\] \[\Rightarrow \] \[\frac{10-u+10+u}{100-{{u}^{2}}}=\frac{20}{91}\] \[\Rightarrow \] \[\frac{20}{100-{{u}^{2}}}=\frac{20}{91}\] \[\Rightarrow \] \[100-{{u}^{2}}=91\] \[\Rightarrow \] \[{{u}^{2}}=9\] \[\Rightarrow \] \[u=3\,km/h\] |
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