Tính bằng cách thuận tiện.

\[\frac{{20}}{{29}}\,\, \times \,\,\frac{4}{5}\,\, \times \,\,29\]

= \[\frac{{4\,\, \times \,\,5\,\, \times \,\,4\,\, \times \,\,29}}{{29\,\, \times \,\,5}}\]

= 16

\[\frac{4}{9}\,\, \times \,\,\frac{5}{{12}}\,\, + \,\,\frac{5}{{12}}\,\, \times \,\,\frac{5}{9}\]

= \[\frac{5}{{12}}\,\, \times \,\,\left( {\frac{4}{9}\,\, + \,\,\frac{5}{9}} \right)\]

= \[\frac{5}{{12}}\,\, \times \,\,1\]

= \[\frac{5}{{12}}\,\]