Решение:

1. $$\frac{4}{6} = \frac{2 cdot 2}{2 cdot 3} = \frac{2}{3}$$
2. $$\frac{15}{12} = \frac{3 cdot 5}{3 cdot 4} = \frac{5}{4}$$
3. $$\frac{70a}{140a} = \frac{70 cdot a}{70 cdot 2 cdot a} = \frac{1}{2}$$
4. $$\frac{35n}{21n} = \frac{7 cdot 5 cdot n}{7 cdot 3 cdot n} = \frac{5}{3}$$