Express the following as $cm$ using decimals.
$2mm$.

 Hint : 1mm=10cm

 As we know ,
$1m = 100cm$ ……(i)
$
  {10^{ - 3}}m = 1mm \\
    \\
 $
As we know the basic is,
$1m = 1000mm$ ……(ii)
$1000mm = 100cm$ {From (i) & (ii)}
On solving above equation we get,
$1mm = 0.1cm$
Here we got the relation between mm and cm.
Now using this relation express 2mm to cm as follows:
$2mm = 0.1 \times 2cm = 0.2cm$
Hence the answer is $0.2cm$.

Note : In these types of questions we have to use the concept of units and measurement. To solve this problem we must know 100 centimeters is equal to 1000 millimeters.Then we can calculate the answer of the given problem by calculating for unit measurement.