What is the percent by mass of nitrogen in ammonium carbonate, ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{CO}}_3$ ?

% nitrogen by mass

  $=\frac{\text{ Mass of nitrogen }}{\text{ Molar mass of ammonium carbonate }}\times 100\mathrm{\%}$

$\frac{14\times 2}{96}\times 100=29.16\mathrm{\%}$.