\[\boxed{\mathbf{551.}\mathbf{\ }}\]
\[1)\ (6 + 14)\ :2 = 20\ :2 = 10\]
\[2)\ (10 + 15)\ :2 = 25\ :2 = 12,5\]
\[3)\ (7 + 9 + 14)\ :3 =\]
\[= 30\ :3 = 10\]
\[4)\ (2 + 3 + 4 + 5)\ :4 =\]
\[= 14\ :4 = 3,5\]