Question

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Q. A person covers a certain distance downstream in 15s and upstream in 30s. How much time will he take to cover the same distance in still water???
(1) 22.5s
(2) 15 x under root 2 s
(3) 20s (Answer)
(4) None of these

Answer 1

formula to find the speed of boat in still water is

1/2(t1+t2)=

1/2(15+30)

1/2(45)

speed of boat in still water is 22.5

Answer 2

Explanation:

First thing you will have to know is that by definition downstream means along the flow of river and upstream means against flow of river

LET speen of man be V1 and speed of river be V2

so when river is supporting motion (downstream )net speed is V1+V2

and  when river is opposing motion (upstream )net speed is V1-V2

we know that speed =distance/time  

also in these cases we consider distance is same so LET be S

so   in upstream S = (V1+V2) *15  

  multiple it by 2 and open the brackets 2S= 30V1+30V2....... 1

and in upstream S = (V1-V2) *30 or S =30V1-30V2 .......2

add 1 and 2 to get

3S=60 V1  

so S/V1 = 20          ........3

now in still water V2= 0

hence time =distance /speed

T =S/V1  

and from equation 3 you get T =20 sec

hope it helps you

brainliest please