How do you balance these equations?
#H_3PO_4 (aq) + Mg(OH)_2(aq) -> Mg_3(PO_4)_2(aq) + H_2O(l)#
#C_4H_10(g) + O_2 (g) -> CO_2 (g) + H_2O(g)#
The same way you balance all equations: conserve mass and charge.
We have thus stoichiometrically balanced an acid-base, and a combustion reaction.
If you like you can double the combustion reaction to remove the non-stoichiometric coefficient in the combustion reaction:
I prefer the former reaction inasmuch as it is easier to specify equivalent masses of hydrocarbon and dioxygen.