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The speed of ...

Question

The speed of motor boat in still water $20 k m / h r$ and river flow is $5 k m / h r$ . A float is dropped from the boat when it start moving upstream. After moving $1.5 k m$ the boat returns back. The boat will catch the float (from initial instant) after

6 m i n

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12 m i n

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10 m i n

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15 m i n

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Solution

The correct option is B $6 m i n$

Speed of boat in upstream = $15 k m / h r$

Distance = $1.5 k m$

Time = $t_{1} = \frac{1.5}{15}$ = $6 m i n$

Distance move by float in this time

$5 \times \frac{1}{10} = \frac{1}{2} k m$

Speed of boat w.r.t. float,

$20 k m / h r, t_{2} = \frac{2 k m}{20}$ = $6 m i n$ .

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