\[\boxed{\text{6.90.}}\]
\[\textbf{а)}\ 10,94 - 2,87 - 1,39 + 0,22 =\]
\[= 8,07 - 1,39 + 0,22 =\]
\[= 6,68 + 0,22 = 6,9\ \]
\[\textbf{б)}\ 28,594 - 18,84 + 2,323 =\]
\[= 9,754 + 2,323 = 12,077\]
\[\textbf{в)}\ 25,98 - (6,92 - 4,27) =\]
\[= 25,98 - 2,65 = 23,33\ \]
\[\textbf{г)}\ 16 - (4,87 + 5,93) =\]
\[= 16 - 10,8 = 5,2\ \]