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\[\boxed{\mathbf{3}.}\]
\[\textbf{а)}\ 8 \cdot (15 - 12)\ :4 + 7 = 13\]
\[\boxed{\mathbf{4}.}\]
\[1.\ а)\ (8 \cdot 9 + 18)\ :(6 - 5) = 90\]
\[\textbf{б)}\ (8 \cdot 9 + 18\ :6) - 5 = 10\]
\[2.\ а)\ 1 \cdot (2 + 3) \cdot 4 \cdot 5 = 100\]
\[\textbf{б)}\ 2 \cdot 3 - 4 + 5 - 6 = 1\]
\[\textbf{в)}\ 3 + 4 - 5 + 6 - 7 = 1\]