Сокращение дробей:

1. $$\frac{78}{338} = \frac{2 \cdot 3 \cdot 13}{2 \cdot 13 \cdot 13} = \frac{3}{13}$$
2. $$\frac{700}{840} = \frac{7 \cdot 100}{7 \cdot 120} = \frac{100}{120} = \frac{10}{12} = \frac{5}{6}$$
3. $$\frac{255}{525} = \frac{5 \cdot 51}{5 \cdot 105} = \frac{51}{105} = \frac{3 \cdot 17}{3 \cdot 35} = \frac{17}{35}$$
4. $$\frac{324}{405} = \frac{9 \cdot 36}{9 \cdot 45} = \frac{36}{45} = \frac{9 \cdot 4}{9 \cdot 5} = \frac{4}{5}$$