11. On the weekend, Xiaohu went boating in the park, departing from the boat rental place at 9:00, and planning to return to the boat rental place by no later than 11:00. It is known that the boat rental place is in the middle of the river, the river channel is straight, and the river current speed is 1.5 kilometers/hour; the boat's speed in still water is 3 kilometers/hour, and while rowing, Xiaohu has to rest for ten minutes after every half hour of rowing, allowing the boat to drift with the current. Question: How far can Xiaohu's boat be from the boat rental place at the farthest?
Just rest for ten minutes and let the boat drift with the current, $120 \div(30+10)=3$, Xiao Hu rests three times, so the boat drifts with the current for 30 minutes. This means that 30 minutes of the time going against the current will be offset, as if the boat hasn't moved. In the remaining $120-30-30=60$ minutes, the boat needs to return to the rental point, so the ratio of the time going with the current to the time going against the current is $V_{\text {with: }} V_{\text {against }}=3: 1$, so the time going with the current is $60 \div(3+1)=15$ minutes. Note that the farthest distance Xiao Hu's boat can be from the rental point also needs to include the distance the boat drifts with the current for 10 minutes,
so the answer is: $4.5 \times 15 \div 60+1.5 \times 10 \div 60=1.375 \text{ km}$