\[\boxed{\text{5.196.}}\]
\[1)\ 90\ 720\ :(207\ :23 \cdot 840) = 12\]
\[207\ :23 = 9\]
\[2)\ 19\ 392\ :48 \cdot (510\ :5) =\]
\[= 41\ 208\ \]
\[510\ :5 = 102\]
\[3)\ 14\ 700\ :21\ :7 \cdot 49 =\]
\[= 700\ :7 \cdot 49 = 100 \cdot 49 =\]
\[= 4\ 900\ \]
\[4)\ 280\ :20\ :(56\ :8)\ :(14\ :7) =\]
\[= 14\ :7\ :2 = 2\ :2 = 1.\ \]