12. A person bought a total of 12 packs of black and white sunflower seeds, spending 9 yuan and 9 jiao. Each pack of white sunflower seeds is 3 jiao more expensive than each pack of black sunflower seeds, and the number of packs of white sunflower seeds is more than that of black sunflower seeds. Question: How many packs of black and white sunflower seeds were bought?
12. Let $x, y$ represent the number of packs of black and white sunflower seeds, respectively, and let the price per pack of black sunflower seeds be $z$ cents. $x+y=12, x z+y(z+3)$ $=99, y>x$. First solve $12 z+3 y=99$. Finally, get $x=3, y=9$, and the price per pack of black sunflower seeds is 6 cents.