в) 1/3 * n + 1/7 * n = 1/4

в) \(\frac{1}{3} n + \frac{1}{7} n = \frac{1}{4}\) \(\frac{7}{21} n + \frac{3}{21} n = \frac{1}{4}\) \(\frac{10}{21} n = \frac{1}{4}\) \(n = \frac{1}{4} : \frac{10}{21}\) \(n = \frac{1}{4} \cdot \frac{21}{10}\) \(n = \frac{21}{40}\)