68. Вычислите: а) \(\frac{7}{12}a+\frac{3}{12}a\), \(a=12\); б) \(\frac{4}{7}a+\frac{6}{7}a\), \(a=7\); в) \(\frac{5}{6}a-\frac{1}{6}a\), \(a=6\); г) \(\frac{1}{55}a+\frac{4}{55}a\), \(a=11\); д) \(\frac{2}{5}a+\frac{3}{5}a\), \(a=2\); е) \(\frac{12}{15}a-\frac{7}{15}a\), \(a=3\); ж) \(\frac{40}{57}a+\frac{17}{57}a\), \(a=1\); з) \(\frac{6}{7}a+\frac{2}{7}a\), \(a=7\); и) \(\frac{3}{7}a+\frac{2}{7}a\), \(a=7\); к) \(\frac{11}{25}a-\frac{6}{25}a\), \(a=5\); л) \(\frac{4}{5}a-\frac{3}{5}a\), \(a=5\); м) \(\frac{7}{11}a+\frac{9}{11}a\), \(a=11\); н) \(\frac{7}{8}a-\frac{5}{8}a\), \(a=4\); о) \(\frac{5}{9}a+\frac{4}{9}a\), \(a=3\).

Складываем или вычитаем коэффициенты при общей букве, затем подставляем значение \(a\).

1. \(\left(\frac{7}{12}+\frac{3}{12}\right)\cdot12=10\).
2. \(\left(\frac{4}{7}+\frac{6}{7}\right)\cdot7=10\).
3. \(\left(\frac{5}{6}-\frac{1}{6}\right)\cdot6=4\).
4. \(\left(\frac{1}{55}+\frac{4}{55}\right)\cdot11=1\).
5. \(\left(\frac{2}{5}+\frac{3}{5}\right)\cdot2=2\).
6. \(\left(\frac{12}{15}-\frac{7}{15}\right)\cdot3=1\).
7. \(\left(\frac{40}{57}+\frac{17}{57}\right)\cdot1=1\).
8. \(\left(\frac{6}{7}+\frac{2}{7}\right)\cdot7=8\).
9. \(\left(\frac{3}{7}+\frac{2}{7}\right)\cdot7=5\).
10. \(\left(\frac{11}{25}-\frac{6}{25}\right)\cdot5=1\).
11. \(\left(\frac{4}{5}-\frac{3}{5}\right)\cdot5=1\).
12. \(\left(\frac{7}{11}+\frac{9}{11}\right)\cdot11=16\).
13. \(\left(\frac{7}{8}-\frac{5}{8}\right)\cdot4=1\).
14. \(\left(\frac{5}{9}+\frac{4}{9}\right)\cdot3=3\).

Ответ: 10; 10; 4; 1; 2; 1; 1; 8; 5; 1; 1; 16; 1; 3.