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А)
$\frac{0.5}{0.4} - \frac{1.23 - 0.98}{ \frac{5}{21} + \frac{1}{3} + \frac{1}{7} } = \\ \frac{ \frac{1}{2} }{ \frac{2}{5} } - \frac{0.25}{ \frac{5}{7} } = \\ \frac{5}{4} - \frac{ \frac{1}{4} }{ \frac{5}{7} } = \\ \frac{5}{4} - \frac{7}{20} = \frac{9}{10} = 0.9$
Б)
$(m - \frac{4}{15} ) \div \frac{4}{5} = 1.5 \\ (m - \frac{4}{15} ) \times \frac{5}{4} = \frac{3}{2} \\ \frac{5}{4} m - \frac{1}{3} = \frac{3}{2} \\ 15m - 4 = 18 \\ 15m = 18 + 4 \\ 15m = 22 \\ m = \frac{22}{15} = 1 \frac{7}{15}$
В)
$2 \frac{1}{3} \times (y - 0.5) = \frac{7}{9} \\ \frac{7}{3} (y - 0.5) = \frac{7}{9} \\ y - 0.5 = \frac{1}{3} \\ y = \frac{1}{3} + 0.5 \\ y = \frac{1}{3} + \frac{1}{2} \\ y = \frac{5}{6}$
Г)
$\frac{0.1}{1.3 - c} = \frac{1}{7} \\ \frac{ \frac{1}{10} }{ \frac{13}{10} - c } = \frac{1}{7} \\ \frac{ \frac{1}{10} }{ \frac{13 - 10c}{10} } = \frac{1}{7} \\ \frac{1}{13 - 10c} = \frac{1}{7} \\ 13 - 10c = 7 \\ - 10c = 7 - 13 \\ - 10c = - 6 \\ c = \frac{ - 6}{ - 10} = 0.6$