This question is incomplete, the complete question is;
Super fine 40-gauge copper wire has a diameter of only 0.080 mm and weighs only 44.5 g/km. Suppose a spool of 40-gauge wire weighs 205 g less after some wire is pulled off to wind a magnet. How could you calculate how much wire was used ?
Answer: length of wire to be used is 4.6067 km
Step-by-step explanation:
Circular diameter = d mm
Linear mass density of wire = μ g/km
let m be the mass of wire un-winded from spool to wind a magnet.
If M is mass of wire initially
then
Δm = M - m { which represents decrease of mass }
Also if L rep the length of wire pulled off,
then
Δm = μ × L grams
⇒ L = Δm/μ km
now given that weight Δm = 205 g and length per unit weight μ = 44.5 g/km
we substitute
L = 205 g / 44.5 g/km
L = 4.6067 km
Therefore length of wire to be used is 4.6067 km