$\frac{9}{10}\times \frac{5}{6}=\frac{9\times 5}{10\times 6}=\frac{3\times \not{3}\times \not{5}}{2\times \not{5}\times 2\times \not{3}}=\frac{3}{4}$
$\frac{6}{25}\div \frac{21}{20}=\frac{6\times 20}{25\times 21}=\frac{2\times \not{3}\times 4\times \not{5}}{5\times \not{5}\times \not{3}\times 7}=\frac{8}{35}$
$\frac{40}{7}\times \frac{14}{5}=\frac{40\times 14}{7\times 5}= \frac{8\times \not{5}\times 2\times \not{7}}{\not{7}\times \not{5}}=\frac{16}{1}= 16$
$\frac{17}{13}\div \frac{51}{26}=\frac{17\times 26}{13\times 51}=\frac{\not{17}\times \not{13}\times 2}{\not{13}\times 3\times\not{17} }=\frac{2}{3}$