Heats of combustion of ${\mathrm{CH}}_4,{\mathrm{C}}_2{\mathrm{H}}_4,{\mathrm{C}}_2{\mathrm{H}}_6\text{ are }-890,-1411\text{ and }-1560\mathrm{kJ}/\mathrm{mole}$ respectively. Which of them has the lowest fuel value in kJ/gm?

The heat combustion of per gm of ${\mathrm{CH}}_4=\frac{890}{16}=-55.625\mathrm{kJ}$
The heat of combustion of per gm of ${\mathrm{C}}_2{\mathrm{H}}_4=\frac{1411}{28}=-50.393\mathrm{kJ}$
The heat of combustion of per gm of ${\mathrm{C}}_2{\mathrm{H}}_6=-\frac{1560}{30}=-52\mathrm{kJ}$