12) 7/10; 1/4; 2/5

$$ \frac{\frac{7}{10} + \frac{1}{4} + \frac{2}{5}}{3} = \frac{\frac{14}{20} + \frac{5}{20} + \frac{8}{20}}{3} = \frac{\frac{27}{20}}{3} = \frac{27}{20} \cdot \frac{1}{3} = \frac{9}{20} $$

Ответ: 9/20