3 a) ∕ ⁄ ∕; б) ∕ − ∕; в) ∕ • ∕; г) 5ª⁻ª− 2⁺⁴⁻⁺ + 3ª.

Решение:

1. \( \frac{5}{9} \div \frac{10}{27} = \frac{5}{9} \times \frac{27}{10} = \frac{5 \times 27}{9 \times 10} = \frac{135}{90} = \frac{3}{2} = 1\frac{1}{2} \)
2. \( \frac{7}{10} - \frac{3}{20} = \frac{14}{20} - \frac{3}{20} = \frac{11}{20} \)
3. \( \frac{6}{25} \times \frac{5}{18} = \frac{6 \times 5}{25 \times 18} = \frac{30}{450} = \frac{1}{15} \)
4. \( 5\frac{2}{7} - 2\frac{4}{21} + 3\frac{2}{3} = \frac{37}{7} - \frac{46}{21} + \frac{11}{3} = \frac{111}{21} - \frac{46}{21} + \frac{77}{21} = \frac{111 - 46 + 77}{21} = \frac{142}{21} = 6\frac{16}{21} \)

Ответ: а) \( 1\frac{1}{2} \); б) \( \frac{11}{20} \); в) \( \frac{1}{15} \); г) \( 6\frac{16}{21} \).