Решение:

1. $$-3,11 \cdot (-4,11) = 12,7821$$
2. $$-0,61 \cdot 13\frac{3}{5} = -0,61 \cdot 13,6 = -8,296$$
3. $$-0,451 : (-0,91) = 0,4956$$
4. $$-121 : (-31) = 3,9032$$
5. $$-5,41 + 11,81 : 0,41 = -5,41 + 28,8049 = 23,3949$$
6. $$4 : \frac{20}{21} + 3\frac{1}{5} \cdot (\frac{3}{4} - 1\frac{3}{16}) = 4 \cdot \frac{21}{20} + \frac{16}{5} \cdot (\frac{3}{4} - \frac{19}{16}) = \frac{21}{5} + \frac{16}{5} \cdot (\frac{12}{16} - \frac{19}{16}) = \frac{21}{5} + \frac{16}{5} \cdot (-\frac{7}{16}) = \frac{21}{5} - \frac{7}{5} = \frac{14}{5} = 2,8$$
7. $$7\frac{3}{4} - (5 - 2\frac{3}{11}):\frac{15}{22} + \frac{1}{8} = \frac{31}{4} - (5 - \frac{25}{11}) : \frac{15}{22} + \frac{1}{8} = \frac{31}{4} - (\frac{55}{11} - \frac{25}{11}) : \frac{15}{22} + \frac{1}{8} = \frac{31}{4} - \frac{30}{11} : \frac{15}{22} + \frac{1}{8} = \frac{31}{4} - \frac{30}{11} \cdot \frac{22}{15} + \frac{1}{8} = \frac{31}{4} - 4 + \frac{1}{8} = \frac{31}{4} - \frac{32}{8} + \frac{1}{8} = \frac{62}{8} - \frac{32}{8} + \frac{1}{8} = \frac{31}{8} = 3,875$$