What length of a copper wire 0.800-mm in a diameter has a resistance of 100 ohm's?

R = rho * L / pi r^2

So L = pi r^2 R / rho

r = 0.400 X 10^-3 m

R = 100 ohm

rho = to be noted in a data book

Plugging these known values we can compute the value of length in meter'