7. Given that box $A$ contains 1 red ball and 5 white balls, and box $B$ contains 3 white balls. Now, 3 balls are randomly taken from box $A$ and placed into box $B$, after which the balls in box $B$ are thoroughly mixed. Then, 3 balls are randomly taken from box $B$ and placed back into box $A$. The probability that the red ball moves from box $A$ to box $B$ and then returns to box $A$ is $\qquad$.
7. 0.25 Take 3 balls from $A$ and put them into $B$, then take 3 balls from $B$ and put them into $A$, there are $C_{6}^{3} \cdot C_{6}^{3}=400$ ways of doing this, while the red balls move from $A$ to $B$, and then from $B$ to $A$ in $C_{5}^{2} \cdot C_{5}^{2}=100$ ways, the required probability is $\frac{100}{400}=0.25$.