Tính (theo mẫu SGK)

a) $\frac{2}{5}\times \frac{9}{8}$ = ............................................................................................................................

$\frac{10}{7}\times \frac{21}{20}$ = ..............................................................................................................................

$6\times \frac{3}{4}$ = ..................................................................................................................................

$\frac{9}{10}\times \frac{7}{3}\times \frac{5}{14}$ = .......................................................................................................................

b) $\frac{6}{5}:\frac{9}{20}$ = .........................................................................................................................

$8:\frac{16}{5}$ = ..................................................................................................................................

$\frac{21}{4}:7$ = ................................................................................................................................

$6\times \frac{1}{12}:\frac{5}{2}$ = ..........................................................................................................................