Решение:

1. a) $$ \frac{5}{12} \div \frac{1}{4} = \frac{5}{12} \times \frac{4}{1} = \frac{5 \times 4}{12 \times 1} = \frac{20}{12} = \frac{5}{3} = 1 \frac{2}{3} $$
2. b) $$ \frac{7}{10} \div \frac{2}{5} = \frac{7}{10} \times \frac{5}{2} = \frac{7 \times 5}{10 \times 2} = \frac{35}{20} = \frac{7}{4} = 1 \frac{3}{4} $$