Express the following lengths in $cm$using decimal:
$\mathit{6}\mathit{ }mm$

If $25 \mathrm{cm} 9 \mathrm{mm}=k\mathit{ }\mathrm{cm}$, then find the value of $k$.

Express the given length in $\mathrm{cm}$using decimal:

$180 \mathrm{mm}=$_____$\mathrm{cm}$