We want to find how many centimeters there are in one kilometer.
We know that one meter is 100 centimeters:
1 m → 100 cm
and 1 kilometer is 1000 meters ('kilo' means 'thousand'):
1 km → 1000 m
Then, the number of centimeter in one kilometer is obtaining by multiplying 100 cm by 1000 m:
1 km → 1000 · 100 cm
1 km → 100,000 cm
