a. $\frac{3}{10}$ x $\frac{4}{9}$ = $\frac{3 x 4}{10 x 9}$ = $\frac{12}{90}$
b. $\frac{6}{5}$ : $\frac{3}{7}$ = $\frac{6}{5}$ x $\frac{7}{3}$ = $\frac{42}{15}$
c. $\frac{3}{4}$ x $\frac{2}{5}$ = $\frac{3 x 2}{4 x 5}$ = $\frac{6}{20}$
d. $\frac{5}{8}$ : $\frac{1}{2}$ = $\frac{5}{8}$ x $\frac{2}{1}$ = $\frac{10}{8}$