Решение:
a) $$rac{3}{7} = rac{3 cdot 12}{7 cdot 12} = rac{36}{84}$$
б) $$rac{15}{17} = rac{15 cdot 8}{17 cdot 8} = rac{120}{136}$$
в) $$rac{12}{23} = rac{12 cdot 4}{23 cdot 4} = rac{48}{92}$$
г) $$rac{10}{11} = rac{10 cdot 13}{11 cdot 13} = rac{130}{143}$$