c: \(\dfrac{2}{21}\cdot\dfrac{9}{4}\cdot\dfrac{7}{3}=\dfrac{7}{21}\cdot\dfrac{9}{3}\cdot\dfrac{2}{4}=\dfrac{1}{2}\)
d: \(\dfrac{5}{9}\cdot\dfrac{3}{8}:\dfrac{15}{16}=\dfrac{5}{8}\cdot\dfrac{3}{9}\cdot\dfrac{16}{15}=\dfrac{5}{15}\cdot\dfrac{16}{8}\cdot\dfrac{1}{3}=\dfrac{2}{9}\)

4 và 5