в) 1\frac{2}{3} \cdot (\frac{1}{3}n + \frac{3}{7}) = 2\frac{1}{4};

в) Решим уравнение:

$$1\frac{2}{3} \cdot (\frac{1}{3}n + \frac{3}{7}) = 2\frac{1}{4}$$

$$\frac{5}{3} \cdot (\frac{1}{3}n + \frac{3}{7}) = \frac{9}{4}$$

$$\frac{1}{3}n + \frac{3}{7} = \frac{9}{4} : \frac{5}{3}$$

$$\frac{1}{3}n + \frac{3}{7} = \frac{9}{4} \cdot \frac{3}{5}$$

$$\frac{1}{3}n + \frac{3}{7} = \frac{27}{20}$$

$$\frac{1}{3}n = \frac{27}{20} - \frac{3}{7}$$

$$\frac{1}{3}n = \frac{27 \cdot 7 - 3 \cdot 20}{140}$$

$$\frac{1}{3}n = \frac{189 - 60}{140}$$

$$\frac{1}{3}n = \frac{129}{140}$$

$$n = \frac{129}{140} : \frac{1}{3}$$

$$n = \frac{129}{140} \cdot \frac{3}{1}$$

$$n = \frac{387}{140} = 2\frac{107}{140}$$

Ответ: $$2\frac{107}{140}$$