length and flow rate

Sorry i forgot to register before posting this question.

Thank you

if a diameter of a pipe is 200mm
the velocity is 0.25m square per second, what is the length of the pipe? I know L=V/Pirexponent 2
But I don't understand how people get 7.958 m when i get 0.0000079577471546

I used to find the cross sectional area: (Pi/4)* D (200^2).
then 0.25 was divided by the result to get 0.0000079577471546.

other gor a positive result 7.958.m
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first i will admit that i have no knowledge to this stuff.
Why does the pipe have to be any particular length? This is a serious question. Can you answer it?
BUT
could it be that the 200 is in mm and the 0.25 is in metres/s ²
so it 200mm is put in as 0.2m I think your quandrey will be overcome.