a. $\frac{9}{10}$ x $\frac{5}{6}$ = $\frac{9 x 5}{10 x 6}$ = $\frac{3 x $\not{3}$ x $\not{5}$}{$\not{5}$ x 2 x $\not{3}$ x 2}$
= $\frac{3}{4}$
b. $\frac{6}{25}$ : $\frac{21}{20}$ = $\frac{6 x 20}{25 x 21}$ = $\frac{2 x $\not{3}$ x 4 x $\not{5}$}{5 x $\not{5}$ x $\not{3}$ x 7}$
= $\frac{8}{35}$
c. $\frac{40}{7}$ x $\frac{14}{5}$ = $\frac{40 x 14}{7 x 5}$ = $\frac{8 x $\not{5}$ x 2 x $\not{7}$}{$\not{5}$ x $\not{7}$}$
= $\frac{16}{1}$ = 16
d. $\frac{17}{13}$ : $\frac{51}{26}$ = $\frac{17 x 26}{13 x 51}$ = $\frac{$\not{17}$ x $\not{13}$ x 2}{$\not{13}$ x 3 x $\not{17}$}$
= $\frac{2}{3}$