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A man rowed $3$ miles upstream in $90$ minutes. If the river flowed with a current of $2$ miles per hour, how long did the man’s return trip take?

  1. $20$ minutes
  2. $30$ minutes
  3. $45$ minutes
  4. $60$ minutes
in Quantitative Aptitude 9.4k points 41 652 824
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30minutes

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Ans is option (B)

Let the speed of the boat be $v$ miles per hour.

While going upstream, river current opposes the boat’s motion. So, the effective speed of the boat would be $(v-2)$ miles per hour.

$\therefore$  $1.5=\frac{3}{v-2}$   $\Rightarrow$   $v=4$ miles per hour.

While going downstream, river current supoorts boat motion. So, the effective speed of the boat would be  $(v+2)=4+2=6$ miles per hour.

$\therefore$  time taken for the downstream trip  $=$  $\frac{3}{6}=0.5$ hours $=$ $30$ minutes

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