\[\boxed{\mathbf{553.}\mathbf{\ }}\]
\[1)\ (4,2 + 2,1)\ :2 = 6,3\ :2 = 3,15\]
\[2)\ (3,9 + 6 + 9,18 + 15,8)\ :4 =\]
\[= (9,9 + 24,98)\ :4 =\]