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Решите задачу:

$1) \frac{3}{4} - \frac{4}{5} = \frac{15}{20} - \frac{16}{20} = - \frac{1}{20}$
$- \frac{1}{20} \times 7.8 = - \frac{1}{20} \times 7 \frac{8}{10} = \\ - \frac{1}{20} \times \frac{78}{10} = - \frac{39}{100}$
$\frac{2}{3} + \frac{4}{7} = \frac{14}{21} + \frac{12}{21} = \frac{26}{21} = 1 \frac{5}{21}$
$1 \frac{5}{21} \times ( - \frac{7}{13} ) = - \frac{26}{21} \times \frac{7}{13} = - \frac{2}{3}$
$- \frac{39}{100 } - ( - \frac{2}{3} ) = - \frac{117}{300} + \frac{200}{300} = \\ \frac{83}{300}$
$\frac{16}{21 } - \frac{1}{12} = \frac{64}{84} - \frac{7}{84} = \frac{57}{84}$

$2) \frac{3}{7} - \frac{16}{21} = \frac{9}{21} - \frac{16}{21} = - \frac{7}{21} = - \frac{1}{3}$
$- \frac{1}{3} \times ( - 2 \frac{2}{7} ) = \frac{1}{3} \times \frac{16}{7} = \frac{16}{21}$
$\frac{11}{15} + \frac{3}{10} = \frac{22}{30} + \frac{9}{30} = \frac{31}{30} = 1 \frac{1}{30}$
$1 \frac{1}{30} \div ( - 12 \frac{2}{5} ) = \frac{31}{30} \div ( - \frac{62}{5} ) = \\ \frac{31}{30} \times ( - \frac{5}{62} ) = - \frac{1}{12}$