A) 6.7kmph
B) 7.5kmph
C) 7.1kmph
D) 7.9kmph
E) 8.3kmph
Correct Answer: B
Solution :
Let the speed of the boat be x kmph Then, speed of the current \[=\frac{x}{3}\] Now, Upstream speed : Downstream speed \[x-\frac{x}{3}\] : \[x+\frac{x}{3}\] \[\frac{2x}{3}\] : \[\frac{4x}{3}\] According to the question, \[\frac{2x}{3}=\frac{20}{4}\] Or, \[\frac{2x}{3}=5\] \[\therefore x=\frac{15}{2}=7.5kmph\] \[\therefore \] Speed of the boat \[\text{= 7}\text{.5 kmph}\] Quicker Approach Upstream speed \[\text{=}\frac{20}{4}=5\text{ kmph}\] Downstream speed \[\text{=}\frac{\text{20}}{\text{2}}\text{=10 kmph}\] \[\therefore \] Speed of the boat\[=\frac{10+5}{2}=\frac{15}{2}\] \[\text{= 7}\text{.5 kmph}\]You need to login to perform this action.
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