Simplify.

a) [[$ \displaystyle\frac {30\;\text{cm} + 5 \;\text{mm}} {5} = $]] cm + mm

b) [[$ \displaystyle\frac {10\;\text{km} + 50 \;\text{m}} {10} = $]] km – m

c) [[$ \displaystyle\frac {12\;\text{kg} + 60 \;\text{g}} {6} = $]] kg + g