6. (10 points) In three consecutive small ponds $A$, $B$, and $C$, several goldfish are placed. If 12 goldfish swim from pond $A$ to pond $C$, then the number of goldfish in pond $C$ will be twice that in pond $A$. If 5 goldfish swim from pond $B$ to pond $A$, then the number of goldfish in ponds $A$ and $B$ will be equal. Additionally, if 3 goldfish swim from pond $B$ to pond $C$, then the number of goldfish in ponds $B$ and $C$ will also be equal. How many goldfish were originally in pond $A$? $\qquad$
【Solution】Solution: If 5 goldfish swim from $B$ to $A$, then $A$ and $B$ will be equal, which means that $B$ has 10 more fish than $A$.

If 3 goldfish swim from $B$ to $C$, then the number of goldfish in $B$ and $C$ will also be equal, indicating that $B$ has 6 more fish than $C$.

Therefore, the number of fish in $A$ is 4 less than in $C$.
If 12 goldfish swim from $A$ to $C$, it means that $C$ has $4+12+12=28$ more fish than $A$. Then the number of goldfish in $C$ will be twice that of $A$. So one part is 28.
There are 28 fish in $A$. Therefore, the original number of goldfish in $A$ is $28+12=40$.
The answer is: 40 fish.