В) 17 c-c 7 c + при с = 3 42 7 18

В) $$42 \frac{1}{7}c - \frac{2}{7}c + \frac{7}{18} c \text{ при } c = 3$$

$$42 \frac{1}{7}c - \frac{2}{7}c + \frac{7}{18} c = c\left(42 \frac{1}{7} - \frac{2}{7} + \frac{7}{18}\right) = c\left(42 + \frac{1}{7} - \frac{2}{7} + \frac{7}{18}\right) = c\left(42 - \frac{1}{7} + \frac{7}{18}\right) = c\left(42 - \frac{1 \cdot 18}{7 \cdot 18} + \frac{7 \cdot 7}{18 \cdot 7}\right) = c\left(42 - \frac{18}{126} + \frac{49}{126}\right) = c\left(42 + \frac{49 - 18}{126}\right) = c\left(42 + \frac{31}{126}\right) = c\left(42 \frac{31}{126}\right)$$

$$\text{Если } c = 3, \text{ то } 42 \frac{31}{126} \cdot 3 = \frac{42 \cdot 126 + 31}{126} \cdot 3 = \frac{5292 + 31}{126} \cdot 3 = \frac{5323}{126} \cdot 3 = \frac{5323 \cdot 3}{126} = \frac{5323}{42} = 126 \frac{31}{42}$$

Ответ: $$126 \frac{31}{42}$$