Решение:
а) $-\frac{1}{5} + \frac{3}{10} - \frac{7}{20} = \frac{-4}{20} + \frac{6}{20} - \frac{7}{20} = \frac{-4 + 6 - 7}{20} = \frac{-5}{20} = \frac{-1}{4};$
б) $-\frac{3}{20} - \frac{7}{30} + \frac{2}{40} = -\frac{3}{20} - \frac{7}{30} + \frac{1}{20} = \frac{-9}{60} - \frac{14}{60} + \frac{3}{60} = \frac{-9 - 14 + 3}{60} = \frac{-20}{60} = -\frac{1}{3};$
в) $\frac{11}{60} - \frac{23}{30} - \frac{17}{20} = \frac{11}{60} - \frac{46}{60} - \frac{51}{60} = \frac{11 - 46 - 51}{60} = \frac{-86}{60} = -\frac{43}{30} = -1\frac{13}{30}.$
Ответ:
а) $\frac{-1}{4};$
б) $-\frac{1}{3};$
в) $-1\frac{13}{30}.$