§1. Действительные числа — 2. Сравнение действительных чисел — 33 — стр. 13

\(12\frac{2}{5}-2\frac{2}{7}:1\frac{19}{21}=12\frac{2}{5}-\frac{16}{7}:\frac{40}{21}=12\frac{2}{5}-\frac{16}{7}\cdot\frac{21}{40}=12\frac{2}{5}-\frac{2}{1}\cdot\frac{3}{5}=12\frac{2}{5}-1\frac{1}{5}=11\frac{1}{5}\).

\(\left(12\frac{2}{5}-2\frac{2}{7}\right):1\frac{19}{21}=\left(12-2+\frac{2\cdot7-2\cdot5}{35}\right):\frac{40}{21}=10\frac{4}{35}\cdot\frac{21}{40}=\frac{354}{35}\cdot\frac{21}{40}=\frac{177}{5}\cdot\frac{3}{20}=\frac{531}{100}=5.31\).