Calculate: \frac{d}{d x}\left(15 x^{\frac{1}{5}}\right) \text { b) } \frac{d}{d a}\left(\frac{x^{6}}{3}+\frac{a^{5}}{2}\right) \text { c) } \frac{d}{d r}\left(3 x^{5}\right) \text { d) } \frac{d}{d t}\left(s t^{2}\right)

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