Question

The speed of a boat in still water is 9kmph and the speed of the stream is 2kmph. Find the total time taken by the boat to go 66km downstream and 35km upstream
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A) 6hrs B) 5hrs C). 11 hrs. D) 1 hr

Answer 1

Answer :

C) 11 hours

Explanation :

Given,

• Speed of the boat in still water, $V_b=9 \ kmph$

• Speed of the stream, $V_w=2 \ kmph$

• Distance covered upstream = 35 km
• Distance covered downstream = 66 km

To find,

• The total time taken by the boat to go 66km downstream and 35km upstream

⇒ Speed of the boat in upstream

            = Speed of boat in still water - speed of the stream

            = $V_b-V_w$

            = 9 - 2

            = 7 kmph

⇒ Speed of the boat in downstream

             = Speed of the boat in still water + speed of the stream

             = $V_b+V_w$

             = 9 + 2

             = 11 kmph

⇒ Time taken by the boat to go 66 km downstream

                   = distance covered downstream/speed of the boat in downstream

                   $=\frac{66 \ km}{11 \ kmph} \\\\ =6 \ hours$

⇒ Time taken by the boat to go 35 km upstream

                   = distance covered upstream/speed of the boat in upstream

                   $=\frac{35 \ km}{7 \ kmph} \\\\ =5 \ hours$

=> Total time taken by the boat to go 66 km downstream and 35 km upstream

               = 6 hours + 5 hours

               = 11 hours