$27\overset{13}{\mathop{\equiv }}\,1\Rightarrow {{(27)}^{1397}}\overset{13}{\mathop{\equiv }}\,{{1}^{1397}}=1$    (1) $2620\overset{13}{\mathop{\equiv }}\,7\Rightarrow (2)\xrightarrow{(2),(1)}{{(27)}^{1397}}+2620\overset{13}{\mathop{\equiv }}\,1+7=8$