\[\boxed{\mathbf{68.}}\]
\[A = \left\{ 2;4\ ;6 \right\};\]
\[\textbf{а)}\ B = \left\{ 3;6 \right\};\]
\[A \cup B = \left\{ 2;3;4;6 \right\};\]
\[P(A \cup B) = \frac{4}{6} = \frac{2}{3}.\]
\[\textbf{б)}\ B = \left\{ 1;3;5 \right\};\]
\[A \cup B = \left\{ 1;2;3;4;5;6 \right\};\]
\[P(A \cup B) = 1.\]
\[\textbf{в)}\ B = \left\{ 4 \right\};\]
\[A \cup B = \left\{ 2;4;6 \right\};\]
\[P(A \cup B) = \frac{3}{6} = \frac{1}{2} = 0,5.\]
\[\textbf{г)}\ B = \left\{ 5 \right\};\]
\[A \cup B = \left\{ 2;4;5;6 \right\};\]
\[P(A \cup B) = \frac{4}{6} = \frac{2}{3}.\]