Question : The speed of the boat down the stream is 125% of the speed in still water. If the boat takes 30 minutes to cover 20 km in still water, then how much time (in hours) will it take to cover 15 km upstream?
Option 1: $\frac{3}{4}$
Option 2: $\frac{1}{2}$
Option 3: $\frac{1}{4}$
Option 4: $1$
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Correct Answer: $\frac{1}{2}$
Solution :
Speed of boat in still water = $\frac{20}{\frac{30}{60}}= 40$ km/hr
Speed of boat downstream = $50$ km/h (As it is $125$% more than the speed of the boat in still water)
Speed of current = $50 - 40$ = $10$ km/hr
The time it will take to cover $15$ km upstream
$=\frac{\text{Distance}}{\text{Speed in still water – speed of current}}=\frac{15}{30} = \frac{1}{2}$ hour
Hence, the correct answer is $\frac{1}{2}$.
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