9、A and B are 66 kilometers apart. Jia and Bing start walking from A to B, while Yi starts walking from B to A. Jia walks 12 kilometers per hour, Yi walks 10 kilometers per hour, and Bing walks 8 kilometers per hour. They all start at the same time. How many hours later will Yi be exactly at the midpoint of the distance between Jia and Bing?
【Analysis】Assume there exists a person Ding, who is always located exactly in the middle of Jia and Bing. At the beginning, Ding is at location $\mathrm{A}$, and Ding's speed is $(12+8) \div 2=10$ kilometers per hour. When Yi and Ding meet, Yi has just walked to the exact middle of Jia and Bing, which takes $66 \div(10+10)=3.3$ hours.