1. Найдите сумму и разность: 1) а) 1/5 + 4/15; б) 2/8 − 5/12; в) 5 1/7 + 7 13/21. 2) а) 3/8 − 1/9; б) 5/6 + 3/11; в) 9 1/20 − 1 3/4. 3) а) 7/30 − 81/45; б) 2 1/20 − 6 9/40; в) 5/18 − 2 23/30.

1. 1)
   а) \(\frac{1}{5}+\frac{4}{15}=\frac{3}{15}+\frac{4}{15}=\frac{7}{15}\).
   б) \(\frac{2}{8}-\frac{5}{12}=\frac{1}{4}-\frac{5}{12}=\frac{3}{12}-\frac{5}{12}=-\frac{1}{6}\).
   в) \(5\frac{1}{7}+7\frac{13}{21}=12+\frac{3}{21}+\frac{13}{21}=12\frac{16}{21}\).
2. 2)
   а) \(\frac{3}{8}-\frac{1}{9}=\frac{27}{72}-\frac{8}{72}=\frac{19}{72}\).
   б) \(\frac{5}{6}+\frac{3}{11}=\frac{55}{66}+\frac{18}{66}=1\frac{7}{66}\).
   в) \(9\frac{1}{20}-1\frac{3}{4}=\frac{181}{20}-\frac{35}{20}=\frac{73}{10}=7\frac{3}{10}\).
3. 3)
   а) \(\frac{7}{30}-\frac{81}{45}=\frac{7}{30}-\frac{9}{5}=\frac{7}{30}-\frac{54}{30}=-\frac{47}{30}=-1\frac{17}{30}\).
   б) \(2\frac{1}{20}-6\frac{9}{40}=\frac{81}{40}-\frac{249}{40}=-\frac{168}{40}=-\frac{21}{5}=-4\frac{1}{5}\).
   в) \(\frac{5}{18}-2\frac{23}{30}=\frac{25}{90}-\frac{203}{90}=-\frac{178}{90}=-\frac{89}{45}=-1\frac{44}{45}\).

Ответ: 1) а) \(\frac{7}{15}\); б) \(-\frac{1}{6}\); в) \(12\frac{16}{21}\). 2) а) \(\frac{19}{72}\); б) \(1\frac{7}{66}\); в) \(7\frac{3}{10}\). 3) а) \(-1\frac{17}{30}\); б) \(-4\frac{1}{5}\); в) \(-1\frac{44}{45}\).