Three varieties of rice, $A, B$ and $C$ costing $? 6 / \mathrm{kg}$, $? 9 / \mathrm{kg}$ and $? 12 / \mathrm{kg}$ are mixed together in a certain ratio. The mixture is sold at $66 \frac{2}{3} \%$ profit for $? 15 / \mathrm{kg}$. Of the total of $100 \mathrm{~kg}$ of the mixture, $50 \mathrm{~kg}$ is variety $B$. Find the quantity of variety $A$ (in $\mathrm{kgs}$ )
(A) 15
(B) 25
(C) 20
(D) 10