[tex] \frac{16}{20} - \frac{17}{18} = \\ \frac{4}{5} - \frac{17}{18} = \\ \frac{72 - 85}{90} = \frac{ - 13}{90} [/tex]
[tex] \frac{3}{16} - \frac{7}{28} + \frac{10}{20} = \\ \frac{3}{16} - \frac{1}{4} + \frac{1}{2} = \\ \frac{3 - 4 + 8}{16} = \\ \frac{7}{16} [/tex]