19. Вычислите: a) 6 1/3 - 8; б) -2 2/7 + 4 3/5; в) 5 1/3 - 6 1/4; г) 3/8 : (-9/16); д) 5/12 * (-6); е) -3 2/9 * 3; ж) 4/7 * (-49); 3) -16 : (-4/9); и) -3 1/2 * (-1 3/7).

Решение:

1. а) \( 6\frac{1}{3} - 8 = \frac{19}{3} - \frac{24}{3} = -\frac{5}{3} = -1\frac{2}{3} \)
2. б) \( -2\frac{2}{7} + 4\frac{3}{5} = -\frac{16}{7} + \frac{23}{5} = \frac{-16 \cdot 5 + 23 \cdot 7}{35} = \frac{-80 + 161}{35} = \frac{81}{35} = 2\frac{11}{35} \)
3. в) \( 5\frac{1}{3} - 6\frac{1}{4} = \frac{16}{3} - \frac{25}{4} = \frac{16 \cdot 4 - 25 \cdot 3}{12} = \frac{64 - 75}{12} = -\frac{11}{12} \)
4. г) \( \frac{3}{8} : \left(-\frac{9}{16}\right) = \frac{3}{8} \cdot \left(-\frac{16}{9}\right) = -\frac{3 \cdot 16}{8 \cdot 9} = -\frac{48}{72} = -\frac{2}{3} \)
5. д) \( \frac{5}{12} \cdot (-6) = -\frac{5 \cdot 6}{12} = -\frac{30}{12} = -\frac{5}{2} = -2\frac{1}{2} \)
6. е) \( -3\frac{2}{9} \cdot 3 = -\frac{29}{9} \cdot 3 = -\frac{29 \cdot 3}{9} = -\frac{87}{9} = -\frac{29}{3} = -9\frac{2}{3} \)
7. ж) \( \frac{4}{7} \cdot (-49) = -\frac{4 \cdot 49}{7} = -4 \cdot 7 = -28 \)
8. з) \( -16 : \left(-\frac{4}{9}\right) = 16 \cdot \frac{9}{4} = \frac{16 \cdot 9}{4} = 4 \cdot 9 = 36 \)
9. и) \( -3\frac{1}{2} \cdot \left(-1\frac{3}{7}\right) = -\frac{7}{2} \cdot \left(-\frac{10}{7}\right) = \frac{7 \cdot 10}{2 \cdot 7} = \frac{70}{14} = 5 \)

Ответ: а) -1 2/3; б) 2 11/35; в) -11/12; г) -2/3; д) -2 1/2; е) -9 2/3; ж) -28; з) 36; и) 5.