[tex]a)[( \frac{9}{28} + \frac{1}{7}) + ( \frac{4}{7} - \frac{1}{28})] \times 7 = \\ \\ \frac{9}{28} + \frac{1}{7} = \frac{9}{28} + \frac{1 \times 4}{7 \times 4} = \frac{9}{28} + \frac{4}{28} = \frac{9 + 4}{28} = \frac{13}{28} \\ \\ \frac{4}{7} - \frac{1}{28} = \frac{4 \times 4}{7 \times 4} - \frac{1}{28} = \frac{16}{28} - \frac{1}{28} = \frac{15}{28} \\ \\ \frac{13}{28} + \frac{15}{28} = \frac{13 + 15}{28} = \frac{28}{28} = 1 \\ \\ 1 \times 7 = 7 [/tex]
