## 10. Math Puzzle 3/66

Two hikers have planned a distance of $40 \mathrm{~km}$. Since one of them has a bicycle, he suggests to the other: "We will take turns using the bicycle, and you will ride for half an hour, then leave the bicycle by the road and continue on foot. In the meantime, I will walk to the bicycle, ride after you until I catch up, then you will ride for another half an hour, etc."

How much time will the two need if they walk at $5 \frac{\mathrm{km}}{\mathrm{h}}$ and cycle at $20 \frac{\mathrm{km}}{\mathrm{h}}$?