Решение:

1. 1) a) 6,83 + 7,81 + 3,17 + 8,19 = (6,83 + 3,17) + (7,81 + 8,19) = 10 + 16 = 26

   б) $$7\frac{1}{4}+13\frac{7}{8}+15\frac{3}{4}+17\frac{1}{8} = (7\frac{1}{4} + 15\frac{3}{4}) + (13\frac{7}{8} + 17\frac{1}{8}) = (7 + 15 + \frac{1}{4} + \frac{3}{4}) + (13 + 17 + \frac{7}{8} + \frac{1}{8}) = (22 + 1) + (30 + 1) = 23 + 31 = 54$$

   Ответ: 54

2. 2) a) 527 - 825 + 925 = 527 + (925 - 825) = 527 + 100 = 627

   б) -5,37 + 9,29 + 4,37 = (4,37 - 5,37) + 9,29 = -1 + 9,29 = 8,29

3. 3) a) $$\frac{2}{19} \cdot 13,5 \cdot 19 = \frac{2}{19} \cdot 19 \cdot 13,5 = 2 \cdot 13,5 = 27$$

   Ответ: 27

   б) $$28 \cdot 3,9 \cdot \frac{5}{14} = 28 \cdot \frac{5}{14} \cdot 3,9 = \frac{28}{14} \cdot 5 \cdot 3,9 = 2 \cdot 5 \cdot 3,9 = 10 \cdot 3,9 = 39$$

   Ответ: 39

4. 4) a) $$\frac{3}{11} \cdot \frac{21}{17} \cdot \frac{11}{3} \cdot \frac{17}{21} = \frac{3}{3} \cdot \frac{11}{11} \cdot \frac{17}{17} \cdot \frac{21}{21} = 1 \cdot 1 \cdot 1 \cdot 1 = 1$$

   Ответ: 1

   б) $$\left(-\frac{7}{31}\right) \cdot \left(-\frac{2}{13}\right) \cdot \frac{31}{7} \cdot \left(-\frac{13}{20}\right) = \left(-\frac{7}{7}\right) \cdot \frac{31}{31} \cdot \left(-\frac{13}{13}\right) \cdot \left(-\frac{2}{20}\right) = (-1) \cdot 1 \cdot (-1) \cdot \left(-\frac{1}{10}\right) = 1 \cdot \left(-\frac{1}{10}\right) = -\frac{1}{10} = -0,1$$

   Ответ: -0,1