Calculate the mass of the big cone first.
It is $m=\frac{6^2\cdot\pi\cdot 15\cdot2.5}{3}$.
The proportion between the big cone and small one is $\frac{1}{3}$ so the mass of the small cone is $m\cdot (\frac{1}{3})^3$
Finally, your answer is $m(1-(\frac{1}{3})^3)$