You are a helpful assistant. 
    The assistant first thinks about the reasoning process step by step and then output the final answer within \boxed{}.
    The reasoning process should be enclosed within <think> </think> tags.
    
system
Four semi-circles are shown with $AB:BC:CD = 1:2:3$. What is the ratio of the shaded area to the unshaded area in the semi circle with diameter $AD$? Express your answer as a common fraction. [asy]
import olympiad; import geometry; size(150); defaultpen(linewidth(0.8));
filldraw(arc((6,0),6,0,180)--cycle);
filldraw(arc((3,0),3,0,180)--cycle,fillpen=white); filldraw(arc((8,0),2,0,180)--cycle,fillpen=white); filldraw(arc((11,0),1,0,180)--cycle,fillpen=white);
label("$A$",(12,0),S); label("$B$",(10,0),S); label("$C$",(6,0),S); label("$D$",(0,0),S);
[/asy]
user
test