Решение:
а) $-\frac{9}{180} - \frac{7}{120} = \frac{-1}{20} - \frac{7}{120} = \frac{-6}{120} - \frac{7}{120} = \frac{-6 - 7}{120} = \frac{-13}{120};$
б) $-\frac{4}{210} + \frac{5}{140} = \frac{-2}{105} + \frac{1}{28} = \frac{-8}{420} + \frac{15}{420} = \frac{-8 + 15}{420} = \frac{7}{420} = \frac{1}{60};$
в) $\frac{-7}{480} + \frac{8}{180} = \frac{-7}{480} + \frac{2}{45} = \frac{-24}{1440} + \frac{64}{1440} = \frac{-21 + 64}{1440} = \frac{43}{1440}.$
Ответ:
а) $\frac{-13}{120};$
б) $\frac{1}{60};$
в) $\frac{43}{1440}.$