3) \frac{2}{15}b + \frac{3}{12}b - \frac{7}{20}b;

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

Ответ: $$\frac{1}{30}b$$