1) 2/15 b + 3/12 b - 7/20 b; 2) 14 5/24 a - 3 7/12 a.

1) $$\frac{2}{15}b + \frac{3}{12}b - \frac{7}{20}b = (\frac{2}{15} + \frac{3}{12} - \frac{7}{20})b = (\frac{2}{15} + \frac{1}{4} - \frac{7}{20})b = (\frac{2 \cdot 4}{15 \cdot 4} + \frac{1 \cdot 15}{4 \cdot 15} - \frac{7 \cdot 3}{20 \cdot 3})b = (\frac{8}{60} + \frac{15}{60} - \frac{21}{60})b = (\frac{8+15-21}{60})b = (\frac{23-21}{60})b = \frac{2}{60}b = \frac{1}{30}b$$

2) $$14 \frac{5}{24}a - 3 \frac{7}{12}a = (14 \frac{5}{24} - 3 \frac{7}{12})a = (14 + \frac{5}{24} - 3 - \frac{7}{12})a = (14-3) + (\frac{5}{24} - \frac{7}{12})a = 11 + (\frac{5}{24} - \frac{7 \cdot 2}{12 \cdot 2})a = 11 + (\frac{5}{24} - \frac{14}{24})a = 11 - \frac{9}{24}a = 11 - \frac{3}{8}a = 10 \frac{5}{8}a$$

Ответ: 1) 1/30 b; 2) 10 5/8 a