The letters a, b, c, d, e, f, and g represent a number from 1 to 7 respectively. If a+b+c=c+d+e=c+f+g, what are the possible values of c? indivual.
字母a，b，c，d，e，f，g分别代表1至7中的一个数字，若a+b+c=c+d+e=c+f+g，则c可取的值有多少个．
<result>7</result>