12. The route from location A to location B consists only of uphill and downhill sections, with a total distance of 21 kilometers. If the speed uphill is 4 kilometers/hour, and the speed downhill is 6 kilometers/hour, the journey from A to B takes 4.25 hours, then the journey from B to A would take ( ) hours.
【Analysis】Assuming the entire route from A $\rightarrow$ B is uphill: $21 \div 4=5.25 \mathrm{~h} \quad 5.25-4.25=1 \mathrm{~h}$ For every $1 \mathrm{~km}$ of downhill, if calculated as uphill, it adds $\frac{1}{4}-\frac{1}{6}=\frac{1}{12}(\mathrm{~h}) \quad 1 \div \frac{1}{12}=12(\mathrm{~km}) \ldots$ Downhill from A $\rightarrow$ B: Uphill $9 \mathrm{~km}$, Downhill $12 \mathrm{~km}$ From B to A: Uphill $12 \mathrm{~km}$, Downhill $9 \mathrm{~km}$, Total time used $12 \div 4+9 \div 6=4.5(\mathrm{~h})$