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[tex]21\frac{1}{5}:\frac{1}{4}=\frac{21\cdot5+1}{5}\cdot4=\frac{105+1}{5}\cdot4=\frac{106}{5}\cdot4=\frac{424}{5}=84\frac{4}{5}\\\\\frac{7}{5}-\frac{47}{42}=4\frac{2}{5}-1\frac{5}{42}=4\frac{84}{210}-1\frac{25}{210}=3\frac{59}{210}\\\\4\frac{6}{7}-\frac{17}{28}=4\frac{24}{28}-\frac{17}{28}=4\frac{7}{28}=4\frac{1}{4}\\\\51\frac{2}{7}-\frac{5}{6}=51\frac{12}{42}-\frac{35}{42}=50\frac{54}{42}-\frac{35}{42}=50\frac{19}{42}[/tex]
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