$1km1000m1km\; \backslash mathop\{\backslash widehat\{=\}\}\; 1000m$ $\Rightarrow \backslash Rightarrow$ $21km=21000m21km=\; 21000m$

$1min601\; min\; \backslash mathop\{\backslash widehat\{=\}\}\; 60$s $\Rightarrow \backslash Rightarrow$ $5min=300s5min=300s$

$1ml\mathrm{0,001}l1ml\; \backslash mathop\{\backslash widehat\{=\}\}\; 0\{,\}001l$ $\Rightarrow \backslash Rightarrow$ $500ml=\mathrm{0,5}l500ml=0\{,\}5l$