Question

15) The speed of boat is 15km/hr A motor boat takes 4 hours 30 minutes to go
30km upstream and downstream to the original point. Find the height of the water

Answer 1

the weight of the water is equally to up an down waard

Answer 2

Let the speed of the steam = x km/hr

Speed of Downstream = (15+x) km/hr

Speed of Upstream = (15-x) km/hr

Time taken to go downstream = 30/(15+x) hours

Time taken to go upstream = 30/(15-x) hours



30/(15-x) + 30/(15+x) = 9/2

10/(15-x) + 10/(15+x) = 3/2 (Divide the numerators by 3 to simply the equation)

[ 10 (15+x) + 10 (15-x) ] / [ (15-x) (15+x) ] = 3/2 — Cross Multiplication

[ 150 + 10x + 150 - 10x ] / [ 225 - x^2 ] = 3/2

300/ 225 - x^2 = 3/2

100/ 225 - x^2 = 1/2

200 = 225 - x^2

x^2 = 25

x = +5 or -5

=> Speed of the stream = 5 km/hr

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