12. 1) \(7\frac{2}{3}a-1,5b-\frac{5}{6}c\) при \(a=\frac{5}{46}\); \(b=-\frac{4}{9}\); \(c=\frac{14}{15}\); 2) \(-\frac{20}{27}a+1\frac{2}{25}b-\frac{8}{39}c\) при \(a=-0,9\); \(b=\frac{5}{9}\); \(c=3,25\); 3) \(4\frac{2}{9}a+8\frac{1}{6}b+\frac{14}{81}c\) при \(a=\frac{3}{19}\); \(b=-3\frac{3}{7}\); \(c=1\frac{13}{14}\); 4) \(1,4a-\frac{51}{92}b+\frac{11}{25}c\) при \(a=\frac{25}{42}\); \(b=1\frac{6}{17}\); \(c=-1\frac{17}{33}\).

1) \(7\frac{2}{3}\cdot\frac{5}{46}-1,5\cdot\left(-\frac{4}{9}\right)-\frac{5}{6}\cdot\frac{14}{15}=\frac{5}{6}+\frac{2}{3}-\frac{7}{9}=\frac{13}{18}\).

2) \(-\frac{20}{27}\cdot(-0,9)+1\frac{2}{25}\cdot\frac{5}{9}-\frac{8}{39}\cdot3,25=\frac{2}{3}+\frac{3}{5}-\frac{2}{3}=\frac{3}{5}\).

3) \(4\frac{2}{9}\cdot\frac{3}{19}+8\frac{1}{6}\cdot\left(-3\frac{3}{7}\right)+\frac{14}{81}\cdot1\frac{13}{14}=\frac{2}{3}-28+\frac{1}{3}=-27\).

4) \(1,4\cdot\frac{25}{42}-\frac{51}{92}\cdot1\frac{6}{17}+\frac{11}{25}\cdot\left(-1\frac{17}{33}\right)=\frac{5}{6}-\frac{3}{4}-\frac{2}{3}=-\frac{7}{12}\).

Ответ: 1) \(\frac{13}{18}\); 2) \(\frac{3}{5}\); 3) \(-27\); 4) \(-\frac{7}{12}\).