a) $\frac{2\times 4}{6\times 18} = \frac{2\times 2\times 2}{2\times 3\times 2\times 9} = \frac{2}{27}$;

b) $\frac{3\times 5\times 7}{6\times 9\times 14} = \frac{3\times 5\times 7}{2\times 3\times 3\times 3\times 2\times 7} = \frac{5}{36}$;

c) $\frac{4\times 7 - 4\times 5}{64} = \frac{4\times(7-5)}{4\times 16} = \frac{4\times 2}{4\times 2\times 8} = \frac{1}{8}$;