з) $$(20 - 19\frac{3}{4}) + (17\frac{3}{4} - 17) + (2\frac{1}{2} - \frac{17}{24}) = \frac{1}{4} + \frac{3}{4} + (2\frac{12}{24} - \frac{17}{24}) = 1 + 2\frac{12}{24} - \frac{17}{24} = 3\frac{12}{24} - \frac{17}{24} = 2\frac{36}{24} - \frac{17}{24} = 2\frac{19}{24}$$.
Ответ: $$2\frac{19}{24}$$