a) \( \dfrac{9}{14} : \left(\dfrac{3}{35} - \dfrac{1}{15} \right) : \dfrac{15}{4} = \)
b) \( \dfrac{15}{21} \cdot \left(\dfrac{5}{4} + \dfrac{1}{12} \right) : \dfrac{5}{14} = \)
c) \( \left(\dfrac{1}{6} - \dfrac{1}{15} \right) : \dfrac{12}{25} : \left(\dfrac{1}{2} + \dfrac{1}{3} \right) = \)
