If you know there are exactly 20 nicks in the 400 metre coil, then the poisson distribution may be inappropriate. This is because the poisson distribution will give a nonzero probability of finding more than 20 nicks in any segment.
however if your teacher told you to use the poisson distribution then I would assume that the expected number of nicks per metre is 0.05. (20/400 =0.05).
The number of nicks (x) in a segment of length (k) is then distributed as
(a)The number of nicks on a segment of length 40 follows a po(0.05*40) = po(2) distribution
Excel will tell you :
in excel 2003, this is POISSON(5,2,1)