Решение:

1. $$\frac{21}{0,6 \cdot 2,8} = \frac{21}{1,68} = \frac{2100}{168} = \frac{1050}{84} = \frac{525}{42} = \frac{175}{14} = \frac{25}{2} = 12,5$$
2. $$\frac{12}{20 \cdot 3} = \frac{12}{60} = \frac{1}{5} = 0,2$$
3. $$\frac{6,9 - 1,5}{2,4} = \frac{5,4}{2,4} = \frac{54}{24} = \frac{27}{12} = \frac{9}{4} = 2,25$$