A. Вычислите действия:

1) $$1.8 + 1\frac{2}{7} = 1.8 + \frac{16}{7} = \frac{18}{10} + \frac{16}{7} = \frac{9}{5} + \frac{16}{7} = \frac{63+80}{35} = \frac{143}{35}$$. $$1.8 - 1\frac{2}{7} = \frac{9}{5} - \frac{16}{7} = \frac{63-80}{35} = -\frac{17}{35}$$. $$\frac{143}{35} : (-\frac{17}{35}) = -\frac{143}{17}$$.
2) $$1.5 + 1\frac{1}{3} = \frac{3}{2} + \frac{4}{3} = \frac{9+8}{6} = \frac{17}{6}$$. $$7.6 - 5\frac{1}{3} = \frac{76}{10} - \frac{16}{3} = \frac{38}{5} - \frac{16}{3} = \frac{114-80}{15} = \frac{34}{15}$$. $$\frac{17}{6} : \frac{34}{15} = \frac{17}{6} \cdot \frac{15}{34} = \frac{1}{2} \cdot \frac{5}{2} = \frac{5}{4}$$.
3) $$23 - 15\frac{1}{3} = 23 - \frac{46}{3} = \frac{69-46}{3} = \frac{23}{3}$$. $$\frac{23}{3} : (-\frac{5}{9}) = \frac{23}{3} \cdot (-\frac{9}{5}) = -\frac{23 \cdot 3}{5} = -\frac{69}{5}$$.
4) $$19 - 13\frac{2}{3} = 19 - \frac{41}{3} = \frac{57-41}{3} = \frac{16}{3}$$. $$\frac{16}{3} : (-7\frac{1}{9}) = \frac{16}{3} : (-\frac{64}{9}) = \frac{16}{3} \cdot (-\frac{9}{64}) = -\frac{1}{1} \cdot \frac{3}{4} = -\frac{3}{4}$$.