Calculate.

a) [[$ \dfrac{15a^8b^7}{5a^3b} = $]]

b) [[$ \dfrac{12c^9d^5}{4c^6d^4} = $]]

c) [[$ \dfrac{6x^9 y^{10} z^5}{30x^2y} = $]]

d) [[$ \dfrac{4a^5b^{11}c^9}{32a^4b^6c^2} = $]]