There are three inlet pipes $A$, $B$, and $C$ in a pool. If only $A$ and $C$ are turned on, the pool can be filled from empty and $18$ tons of water are from $A$. If only $B$ and $C$ are turned on, the pool can be filled from empty and $27$ tons of water are from $B$. Given the flow rate of $B$ is twice that of $A$, how many tons of water can the pool be filled up with? 
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