a) $$\frac{15}{17} \times \frac{2}{3} = \frac{15 \times 2}{17 \times 3} = \frac{5 \times 2}{17 \times 1} = \frac{10}{17}$$
б) $$\frac{4}{9} \times \frac{3}{5} = \frac{4 \times 3}{9 \times 5} = \frac{4 \times 1}{3 \times 5} = \frac{4}{15}$$
в) $$\frac{21}{32} \times \frac{8}{9} = \frac{21 \times 8}{32 \times 9} = \frac{7 \times 1}{4 \times 3} = \frac{7}{12}$$
г) $$\frac{8}{35} \times \frac{15}{32} = \frac{8 \times 15}{35 \times 32} = \frac{1 \times 3}{7 \times 4} = \frac{3}{28}$$
Ответ: a) $$\frac{10}{17}$$, б) $$\frac{4}{15}$$, в) $$\frac{7}{12}$$, г) $$\frac{3}{28}$$