A horita voy a poner un ejemplo.
 $${\mathtt{a}}{\mathtt{\,\small\textbf+\,}}{\mathtt{b}} = {\mathtt{c}}$$ 
 $${\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\mathtt{c}} = {\mathtt{a}}$$
 $${\mathtt{a}} = {\mathtt{1}}$$
 $${\mathtt{b}} = {\mathtt{2}}$$
 $${\mathtt{c}} = {\mathtt{1}}$$
 A hora, suma a+b=c.
 $${\mathtt{a}}{\mathtt{\,\small\textbf+\,}}{\mathtt{b}} = {\mathtt{c}} = {\mathtt{1}}{\mathtt{\,\small\textbf+\,}}{\mathtt{2}} = {\mathtt{3}}$$
 Entonces, $${\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\mathtt{c}} = {\mathtt{a}} = {\mathtt{2}}{\mathtt{\,\small\textbf+\,}}{\mathtt{1}} = {\mathtt{3}}$$
 Conclusion:
 Por eso, C y A son los mismos numeros, y sumas B, hace tu respuesta.