Answer
The change from one unit to another in the metric system can be done using the shifting of decimals only because the units in the metric system are in multiple of ten.
Work Step by Step
To change the units within a metric system there is no need for any unit fractions. The conversion from one unit to another can be done easily by shifting the decimal point only.
For example: In the Metric system: There are \[10\] millimeters in \[1\] centimeter, \[10\] centimeters in \[1\] decimeter, \[10\] decimeters in one meter, \[10\] meters in one decameter, \[10\]decameters in one hectometer and \[10\]hectometers in one kilometer.
The conversion from meter to kilometer includes three steps where for each step the given term is divided by\[10\]. Hence for three steps,\[4200\]meters shall be divided by \[1000\] to get the result in a kilometer.
Hence, it is required to move the decimal place to three places left:
\[\text{4,200 m}=4.2\text{ km}\]