\[\boxed{\mathbf{284.}}\]
\[P(a + b) = 0,4;\]
\[P(a + c) = 0,7;\]
\[P(a + b) + P(c) = 1;\]
\[0,4 + P(c) = 1\]
\[P(a + c) + P(b) = 1\]
\[0,7 + P(b) = 1\]
\[P(a) + P(b) + P(c) = 1\]
\[P(a) + 0,3 + 0,6 = 1\]