Найдите значение выражений

16)
$( \frac{2}{3} - \frac{3}{7} ) \div \frac{10}{21} = ( \frac{14}{21} - \frac{9}{21} ) \div \frac{10}{21} = \frac{5}{21} \div \frac{10}{21} = \frac{5}{21} \times \frac{21}{10} = \frac{1}{2} = 0.5$
17)
$( \frac{3}{4} - \frac{2}{3} ) \div \frac{5}{6} = ( \frac{9}{12} - \frac{8}{12} ) \div \frac{5}{6} = \frac{1}{12} \div \frac{5}{6} = \frac{1}{12} \times \frac{6}{5} = \frac{1}{2 \times 5} = \frac{1}{10} = 0.1$
18)
$( \frac{1}{7} + \frac{1}{8} ) \div \frac{5}{28} = ( \frac{8}{56} + \frac{7}{56} ) \div \frac{5}{28} = \frac{15}{56} \div \frac{5}{28} = \frac{15}{56} \times \frac{28}{5} = \frac{3 \times 1}{2 \times 1} = \frac{3}{2} = \frac{15}{10} = 1.5$
19)
$( \frac{5}{6} - \frac{3}{7} ) \div \frac{17}{21} = ( \frac{35}{42} - \frac{18}{42} ) \div \frac{17}{21} = \frac{17}{42} \div \frac{17}{21} = \frac{17}{42} \ \times \frac{21}{17} = \frac{1}{2} = 0.5$