\[\boxed{\mathbf{3.}\mathbf{80}}\]
\[\textbf{а)}\ 91\ :7 = (70 + 21)\ :7 =\]
\[= 70\ :7 + 21\ :7 = 10 + 3 = 13\]
\[\textbf{б)}\ 216\ :18 = (180 + 36)\ :18 =\]
\[= 180\ :18 + 36\ :18 = 10 + 2 =\]
\[= 12\]
\[\textbf{в)}\ 6817\ :17 =\]
\[= (6800 + 17)\ :17 =\]
\[= 6800\ :17 + 17\ :17 =\]
\[= 400 + 1 = 401\]
\[\textbf{г)}\ 240\ 824\ :8 =\]
\[= (240\ 000 + 800 + 24)\ :8 =\]
\[= 240\ 000\ :8 + 800\ :8 + 24\ :8 =\]
\[= 30\ 000 + 100 + 3 = 30\ 103\]