Решим уравнение:
$$1\frac{2}{3} \cdot (\frac{1}{3} n + \frac{3}{7}) = 2\frac{1}{4}$$.
$$\frac{5}{3} \cdot (\frac{1}{3} n + \frac{3}{7}) = \frac{9}{4}$$.
$$\frac{1}{3} n + \frac{3}{7} = \frac{9}{4} : \frac{5}{3} = \frac{9}{4} \cdot \frac{3}{5} = \frac{27}{20}$$.
$$\frac{1}{3} n = \frac{27}{20} - \frac{3}{7} = \frac{189}{140} - \frac{60}{140} = \frac{129}{140}$$.
$$n = \frac{129}{140} : \frac{1}{3} = \frac{129}{140} \cdot 3 = \frac{387}{140} = 2\frac{107}{140}$$.
Ответ: 387/140