6. The teacher fills some numbers into the circles in the figure below (each circle can and can only contain one number). The sum of the three numbers in each of the two closed loops on the left and right is 30, and the sum of the four numbers in each of the two closed loops on the top and bottom is 40. If the number in circle $X$ is 9, then the number in circle $Y$ is $\qquad$
【Answer】 11
【Solution】As shown in the figure, $\left\{\begin{array}{l}a+b+c+d=40 \\ X+Y+c+b=40\end{array} \Rightarrow(a+b+c+d+X+Y)+c+b=80\right.$, $\left\{\begin{array}{l}a+b+X=30 \\ c+d+Y=30\end{array} \Rightarrow a+b+c+d+X+Y=60\right.$, we deduce that $c+b=20$. Substituting $c+b=20$ into $X+Y+c+b=40 \Rightarrow X+Y=20$. Since $X=9$, then $Y=11$.