Số 20 450 197 được đọc là:  

Đáp án đúng là: D

\(\frac{{123}}{{17}}\,\, - \,\,\left( {\frac{5}{{17}}\,\, - \,\,\frac{6}{{51}}} \right)\,\, = \,\,\frac{{123}}{{17}}\,\, - \,\left( {\frac{5}{{17}}\,\, - \frac{2}{{17}}} \right) = \frac{{123}}{{17}}\,\, - \frac{3}{{17}} = \frac{{120}}{{17}}\)

a) \(\frac{{49}}{{35}}\) và \(\frac{{12}}{{32}}\)

Rút gọn:

\(\frac{{49}}{{35}}\,\, = \,\,\frac{{49\,\,:\,\,7}}{{35\,\,:\,\,7}}\,\, = \,\,\frac{7}{5}\);      \(\frac{{12}}{{32}}\,\, = \,\,\frac{{12\,\,:\,\,4}}{{32\,\,:\,\,4}}\,\, = \,\,\frac{3}{8}\)

Quy đồng mẫu số:

\(\frac{7}{5}\,\, = \,\,\frac{{7\,\, \times \,\,8}}{{5\,\, \times \,\,8}}\,\, = \,\,\frac{{56}}{{40}}\);              \(\frac{3}{8}\,\, = \,\,\frac{{3\,\, \times \,\,5}}{{8\,\, \times \,\,5}}\,\, = \,\,\frac{{15}}{{40}}\)

b) \(\frac{{20}}{{35}}\) và \(\frac{6}{{24}}\)

Rút gọn:

\(\frac{{20}}{{35}}\,\, = \,\,\frac{{20\,\,:\,\,5}}{{35\,\,:\,\,5}}\,\, = \,\,\frac{4}{7}\)

\(\frac{6}{{24}}\,\, = \,\,\frac{{6\,\,:\,\,6}}{{24\,\,:\,\,6}}\,\, = \,\,\frac{1}{4}\)

Quy đồng mẫu số:

\(\frac{4}{7}\,\, = \,\,\frac{{4\,\, \times \,\,4}}{{7\,\, \times \,\,4}}\,\, = \,\,\frac{{16}}{{28}}\)

\(\frac{1}{4}\,\, = \,\,\frac{{1\,\, \times \,\,7}}{{4\,\, \times \,\,7}}\,\, = \,\,\frac{7}{{28}}\)