CBSE 10th Class Mathematics

Past year question paper

Year 2020

SET 1, Code Number – 30/5/1

Question Number – 37 [b]

speed, distance and time problem

use the relation of speed, distance and time

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# A motor boat whose speed is 18km/h in still water takes 1 hour more to go 24km upstream than to return downstream to the same spot. Find the speed of the stream.

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This answer was edited.Speed of boat = 18 km/h

Let speed of the stream be =x km/h

Speed of upstream = ( 18 – x ) km/hr

Speed of downstream = ( 18 + x ) km/hr

Distance = 24 km

Time = Distance / Speed

As per question,

[24 / (18 – x)] – [24 / (18 + x)] = 1

24 { [1 / (18 – x)] – [1 / (18 + x) ] } = 1

[2x/(324 – x²)] = 1 / 24

324 – x² = 48

x² + 48x -324 = 0

[x +54] [x – 6] = 0

x = -54, x = 6

x = 6 km / hr

∴ Speed of stream =6 km / hr