Ответ:

  1. \(\frac{12}{18}=\frac{12\div6}{18\div6}=\frac23\).
  2. \(\frac6{54}=\frac{6\div6}{54\div6}=\frac19\).
  3. \(\frac{56}{98}=\frac{56\div14}{98\div14}=\frac47\).
  4. \(\frac{42\cdot24}{8\cdot21}=\frac{42}{21}\cdot\frac{24}{8}=2\cdot3=6\).