\(a,\dfrac{21\times15\times9}{7\times45\times18}=\dfrac{3\times7\times15\times9}{7\times15\times3\times9\times2}=\dfrac{1}{2}\\ b,\dfrac{24\times11\times27}{33\times8\times12\times9}=\dfrac{12\times2\times11\times3\times9}{11\times3\times8\times2\times6\times9}=\dfrac{12}{8\times6}=\dfrac{12}{48}=\dfrac{1}{4}\)