Question

A man rows to a place 40 km distant and
back in a total of 18 hours. He finds that he
can row 5 km with the stream in the same
time as 4 km against the stream. What is the
speed of boat in still water?​

Answer 1

Step-by-step explanation:

Let the speed of the boat be x and the speed of the stream be y.

While going 40 km upstream,

effective speed of the boat = x+ y

t1 = time taken = 40/(x + y)

While going 40 km downstream,

effective speed of the boat = x - y

t2 = time taken = 40/(x - y)

t1 + t2 = 9

=> 40/(x + y) + 40/(x - y) = 9 → equation 1

t3 = time taken to row downstream 5 km = 5/(x + y)

t4 = time taken to row upstream 4 km = 4/(x - y)

t3 = t4

=> 5/(x + y) = 4/(x - y)

=> 5(x - y) = 4(x + y)

=> 5x - 5y = 4x + 4y

=> x = 9y

Substituting this value of x in equation 1,

40/(9y + y) + 40/(9y - y) = 9

=> 40/10y + 40/8y = 9

=> 4/y + 5/y = 9

=> y = 1

The speed of the stream is 1 km/h.

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Answer 2

Answer:

speed of boat in still water for 18 hours question

5.5 km/hr