Express the following as cm using decimals.
$4 cm$ $2 mm$

Hint: In this question we have to convert $4cm$ $2mm$ in $cm$, so the key concept is to express the $2mm$ in $cm$ only by using the conversion rule of length and then by adding this part to $4cm$ part we get the right answer.

Complete step-by-step answer:
We have been given that $4cm2mm$.
Now convert the $2mm$ into $cm$.
We know that $1cm = 10mm$
Or, $1mm = \dfrac{1}{{10}}cm$
Or, $1mm = 0.1cm$
So, $2mm = 2 \times 0.1cm$
Or, $2mm = 0.2cm$
Hence we can write $4cm 2 mm = 4cm + 0.2cm = 4.2cm$.

Note: Whenever we face such types of problems the key point is to convert the whole equations in the same unit by using the conversion scale. So we should remember the conversion scale of all the fundamental units to get the right answer.