Решение:
a) $$5:15 = 4:12$$
- $$5:4 = 15:12$$
- $$12:15 = 4:5$$
- $$12:4 = 15:5$$
б) $$\frac{12}{0,2} = \frac{30}{0,5}$$
- $$\frac{12}{30} = \frac{0,2}{0,5}$$
- $$\frac{0,5}{0,2} = \frac{30}{12}$$
- $$\frac{0,5}{30} = \frac{0,2}{12}$$
в) $$\frac{m}{n} = \frac{l}{k}$$
- $$\frac{m}{l} = \frac{n}{k}$$
- $$\frac{k}{n} = \frac{l}{m}$$
- $$\frac{k}{l} = \frac{n}{m}$$