Решение:

Решим по действиям:

1. $$3 - 1\frac{20}{21} = 3 - \frac{41}{21} = \frac{63}{21} - \frac{41}{21} = \frac{22}{21}$$
2. $$\frac{22}{21} : \frac{4}{49} = \frac{22}{21} \cdot \frac{49}{4} = \frac{22 \cdot 49}{21 \cdot 4} = \frac{11 \cdot 7}{3 \cdot 2} = \frac{77}{6} = 12\frac{5}{6}$$
3. $$13 - 12\frac{5}{6} = 13 - \frac{77}{6} = \frac{78}{6} - \frac{77}{6} = \frac{1}{6}$$
4. $$\frac{1}{6} + \frac{3}{10} = \frac{5}{30} + \frac{9}{30} = \frac{14}{30} = \frac{7}{15}$$

Ответ: $$\frac{7}{15}$$