1. Zadatak

$$a = \frac73 b,\ b = \frac52 c$$ $$a = \frac73 b = \frac73 \cdot \frac52 c = \frac{35}{6}c$$ $$\begin{aligned} (a-b):(b+c) &= \left(\frac{35}{6}c - \frac52 c\right):\left(\frac52 c+c\right) \\ &= \left(\frac{10}{3}c\right):\left(\frac72 c\right) = 20:21 \end{aligned}$$