0.72
解:动点从A1点出发走三步到达C点(设发生堵塞的概率为P),共有6种走法:
①A1→A→B→C,此时P=1-(1-0.2)(1-0.3)(1-0.5)=0.72;
②A1→A→D→C,此时P=1-(1-0.2)(1-0.3)(1-0.6)=0.776;
③A1→B1→B→C,此时P=1-(1-0.4)(1-0.3)(1-0.5)=0.79;
④A1→B1→C1→C,此时P=1-(1-0.4)(1-0.6)(1-0.4)=0.856;
⑤A1→D1→D→C,此时P=1-(1-0.1)(1-0.4)(1-0.6)=0.784;
⑥A1→D1→C1→C,此时P=1-(1-0.1)(1-0.5)(1-0.4)=0.73,综上可知,走法①发生堵塞的概率最小.
答案:0.72.