1) 7/16 : 8/21;

1) $\frac{7}{16} \cdot \frac{8}{21} = \frac{7}{21} \cdot \frac{8}{16} = \frac{1}{3} \cdot \frac{1}{2} = \frac{1}{6}$ 2) $-\frac{5}{11} \cdot \frac{33}{65} = -\frac{5}{65} \cdot \frac{33}{11} = -\frac{1}{13} \cdot 3 = -\frac{3}{13}$ 3) $\frac{19}{23} \cdot \left(-\frac{46}{57}\right) = -\left(\frac{19}{57} \cdot \frac{46}{23}\right) = -\left(\frac{1}{3} \cdot 2\right) = -\frac{2}{3}$ 4) $3\frac{1}{4} \cdot 3\frac{9}{13} = \frac{13}{4} \cdot \frac{48}{13} = \frac{1}{1} \cdot \frac{12}{1} = 12$ 5) $-5\frac{1}{3} \cdot 1\frac{1}{2} = -\frac{16}{3} \cdot \frac{3}{2} = -\frac{16}{2} = -8$ 6) $-3\frac{1}{7} \cdot 1\frac{3}{11} = -\frac{22}{7} \cdot \frac{14}{11} = -\left(\frac{22}{11} \cdot \frac{14}{7}\right) = -(2 \cdot 2) = -4$ 7) $\frac{5}{7} : \left(-\frac{10}{21}\right) = \frac{5}{7} \cdot \left(-\frac{21}{10}\right) = -\left(\frac{5}{10} \cdot \frac{21}{7}\right) = -\left(\frac{1}{2} \cdot 3\right) = -1,5$ 8) $-\frac{4}{5} : \left(-1\frac{1}{15}\right) = -\frac{4}{5} : \left(-\frac{16}{15}\right) = \frac{4}{5} \cdot \frac{15}{16} = \frac{1}{1} \cdot \frac{3}{4} = \frac{3}{4} = 0,75$ 9) $31\frac{1}{2} : 2\frac{1}{31} = \frac{63}{2} : \frac{63}{31} = \frac{63}{2} \cdot \frac{31}{63} = \frac{31}{2} = 15,5$ 10) $\frac{5}{9} \cdot (-9) = \frac{5}{9} \cdot \left(-\frac{9}{1}\right) = -5$ 11) $-\frac{4}{5} : 9 = -\frac{4}{5} \cdot \frac{1}{9} = -\frac{4}{45}$ 12) $-14 : \left(-2\frac{4}{5}\right) = -14 : \left(-\frac{14}{5}\right) = 14 \cdot \frac{5}{14} = 5$ 13) $6,5 \cdot 2,6 = 16,9$ 14) $-5,3 \cdot 7,7 = -40,81$ 15) $-6,4 \cdot (-1,3) = 8,32$ 16) $0,81 : 0,009 = 810 : 9 = 90$ 17) $0,1515 : 0,05 = 15,15 : 5 = 3,03$ 18) $0,361 : 0,19 = 36,1 : 19 = 1,9$