.
First sentence of the problem
"Wei owns a pet store and wishes to buy at least 8 cats and 10 dogs from a breeder. "
can be read and interpret in two different ways.
One way is
"Wei owns a pet store and wishes to buy at least 8 cats and at least 10 dogs from a breeder."
Another way is
"Wei owns a pet store and wishes to buy at least 8 cats and exactly 10 dogs from a breeder."
These different interpretations imply different equations/inequalities.
So, chose what you really want, and then re-post the updated version to the forum.
Please do not post it to me personally.