Question

a sailor goes 8 km downstream in 40 minutes and returns in 1 hour find the speed of sailor in still water and
the speed of current

Answer 1

Here is your answer 
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Let the speed of sailor in still water is x km/hr and the speed of the stream is y km/hr.

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=> 8/(x + y) = 4/6

=> 4(x + y) = 8*6

=> 4(x + y) = 48

=> x + y = 48/4

=> x + y = 12   ................1

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8/(x - y) = 1

x - y = 8  ...............2

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Add equation 1 and 2, we get

    2x = 20

=> x = 20/2

=> x = 10

From equation 1, we get

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 =>   10 + y = 12

=> y = 12 - 10

=> y = 2

Hence, the speed of sailor in still water is 10 km/hr and the speed of stream is 2 km/hr

Answer 2

Let the speed of sailor in still water is x km/hr and the speed of stream is y km/hr.

Now, the speed of boat (upstream) = (x - y) km/hr

and the speed of boat (downstream) = (x + y) km/hr

Now, according to question,

8/(x + y) = 40/60 {Since time = distance/speed}

=> 8/(x + y) = 4/6

=> 4(x + y) = 8*6

=> 4(x + y) = 48

=> x + y = 48/4

=> x + y = 12 ................1

Again, 8/(x - y) = 1

x - y = 8 ...............2

Add equation 1 and 2, we get

2x = 20

=> x = 20/2

=> x = 10

From equation 1, we get

10 + y = 12

=> y = 12 - 10

=> y = 2