11. We define: $a @ b=a \times(a+1) \times \ldots \times(a+b-1)$. Given that $x @ y @ 2=420$, then $y @ x=(\quad)$
【Analysis】Let $\mathrm{x} @ \mathrm{y}=\mathrm{m}$, then $\mathrm{m} @ 2=420=\mathrm{m} \times(m+1) \Rightarrow m=20$
That is
$x @ y=20=4 \times 5 \Rightarrow x=4, y=2$
$x @ y=20=1 \times 20=4 \times 5$
$x=20, y=1$ or $x=4, y=2$
Then $y$ @ $x=1 @ 20=20$ !; or $y @ x=2 @ 4=2 \times 3 \times 4 \times 5=120$
Answer: 20 or 120