7. (6 points) $A$ and $B$ two buckets of water weigh the same. If 2.5 kilograms of water are poured from bucket $A$ to bucket $B$, then the weight of the water in bucket $B$ is 6 times the weight of the water in bucket $A$. How much water was originally in bucket $B$? $\qquad$ kilograms.
【Solution】Solution: $2.5 \times 2 \div(6-1)+2.5$
$$
\begin{array}{l}
=5 \div 5+2.5 \\
=1+2.5 \\
=3.5 \text { (kg) }
\end{array}
$$

Answer: Bucket $B$ originally contained 3.5 kg of water.
Therefore, the answer is: 3.5.