3. Вычислите: 1) 3/8 + 4/9; 2) 7/12 − 3/8; 3) 2 5/8 + 1 3/10; 4) 6 7/20 − 4 5/12.

1. НОК(8, 9) = 72: \(\frac{3}{8}+\frac{4}{9}=\frac{27}{72}+\frac{32}{72}=\frac{59}{72}\).
2. НОК(12, 8) = 24: \(\frac{7}{12}-\frac{3}{8}=\frac{14}{24}-\frac{9}{24}=\frac{5}{24}\).
3. \(2\frac{5}{8}+1\frac{3}{10}=3+\frac{25}{40}+\frac{12}{40}=3\frac{37}{40}\).
4. Займём единицу: \(6\frac{7}{20}=5\frac{27}{20}\). Тогда \(5\frac{27}{20}-4\frac{5}{12}=1+\frac{81}{60}-\frac{25}{60}=2\frac{56}{60}=2\frac{14}{15}\).

Ответ: 1) \(\frac{59}{72}\); 2) \(\frac{5}{24}\); 3) \(3\frac{37}{40}\); 4) \(2\frac{14}{15}\).