In order to determine how many millimeters the spool has, you take into account the following identity:
1 meter = 1000 millimeters (1 m = 1000mm)
Then, you can proceed as follow:
100 m = 100(1m) = 100 (1000 mm) = 100,000 mm
where you have multiplied by 1000 the given quantity in meters, that is, you replace the unit meter (m) by its equivalence in millimeters (1000 mm)
That is, 100 meters are equivalent to 100,000 millimeters
