Решение:

1. a) $$rac{5}{8} \times \frac{8}{9} = \frac{5 \times 8}{8 \times 9} = \frac{5 \times \cancel{8}}{\cancel{8} \times 9} = \frac{5}{9}$$

2. b) $$rac{7}{12} \times \frac{3}{14} = \frac{7 \times 3}{12 \times 14} = \frac{\cancel{7} \times \cancel{3}}{4 \times \cancel{3} \times 2 \times \cancel{7}} = \frac{1}{4 \times 2} = \frac{1}{8}$$

3. c) $$rac{3}{4} \times 20 = \frac{3 \times 20}{4} = \frac{3 \times 5 \times \cancel{4}}{\cancel{4}} = 3 \times 5 = 15$$

4. e) $$rac{4}{9} : \frac{1}{3} = \frac{4}{9} \times \frac{3}{1} = \frac{4 \times 3}{9 \times 1} = \frac{4 \times \cancel{3}}{3 \times \cancel{3} \times 1} = \frac{4}{3} = 1\frac{1}{3}$$

5. f) $$2\frac{1}{2} \times \frac{7}{10} = \frac{2 \times 2 + 1}{2} \times \frac{7}{10} = \frac{5}{2} \times \frac{7}{10} = \frac{5 \times 7}{2 \times 10} = \frac{\cancel{5} \times 7}{2 \times 2 \times \cancel{5}} = \frac{7}{4} = 1\frac{3}{4}$$

6. g) $$3\frac{1}{3} \times 1\frac{4}{5} = \frac{3 \times 3 + 1}{3} \times \frac{1 \times 5 + 4}{5} = \frac{10}{3} \times \frac{9}{5} = \frac{10 \times 9}{3 \times 5} = \frac{2 \times \cancel{5} \times 3 \times 3}{ \cancel{3} \times \cancel{5}} = 2 \times 3 = 6$$