Question

difference between km to dm to cm to mm

Answer 1

An simple way to look at the conversion without confusing people with powers of 10:
\[(1 km)(\frac{1000m}{1 km})(\frac{10 dm}{1 m})\]

Answer 2

So, you can look at how many decameters go into 1 kilometer, and use brain power to write a direct conversion with powers of 10.