Question

find the area of each of the following fields. All dimensions are in metres.
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Answer 1

Let speed of stream be 's'

Let speed of boat in still water be 'b'

Speed of boat in still water = b = 5 km/h

Let time taken by the boat in going 40km downstream be t1 and time taken by the boat in going 40km upstream be t2.

Time = distance/speed

t1 = 40/(b+s)

t2 = 40/(b-s)

He takes thrice as much as time going in 40km upstream as going 40km downstream.

→t2 = 3t1

→40/(5-s) = 3(40/(5+s))

→1/(5-s) = 3[1/(5+s)]

→5+s = 3(5-s)

→5+s = 15-3s

→ s+3s = 15-5

→ 4s = 10

→ s = 10/4

→ s = 5/2