[tex]a) \ 21\frac{1}{5}:\frac{1}{4} = \frac{106}{5}\cdot\frac{4}{1} =\frac{424}{5} = 84\frac{4}{5}[/tex]
[tex]b) \ 3\frac{7}{5}-\frac{47}{42} = 3\frac{7}{5} - 1\frac{5}{42} = 3\frac{294}{210} - 1\frac{25}{210} = 2\frac{269}{210} =3\frac{59}{210}[/tex]
[tex]c) \ 4\frac{6}{7}-\frac{17}{28} = 4\frac{24}{28}-\frac{17}{28} = 4\frac{7}{28} = 4\frac{1}{4}[/tex]
[tex]d) \ 4\frac{6}{7}-\frac{17}{28} = 4\frac{24}{28} -\frac{17}{28} = 4\frac{7}{28} = 4\frac{1}{4}[/tex]
[tex]f) \ 51\frac{2}{7}-\frac{5}{6} = 50\frac{9}{7}-\frac{5}{6} = 50\frac{63}{42} - \frac{35}{42} = 50\frac{19}{42}[/tex]