3. A cyclist is moving from place A to place B, and halfway between A and B is place C. The cyclist covered the distance from A to C in 4 hours, traveling at a speed of $18 \mathrm{~km}$ per hour. How many hours will it take the cyclist to cover the distance from place C to B, if he travels at a speed of $12 \mathrm{~km}$ per hour?
Solution. The distance from place A to place C is $4 \cdot 18=72 \mathrm{~km}$. From the condition of the problem, the distance from $\mathrm{C}$ to $\mathrm{B}$ is the same as the distance from A to C, so the distance from C to B is $4 \cdot 18=72 \mathrm{~km}$. Since the traveler moved at a speed of $12 \mathrm{~km}$ per hour, we have $12 \cdot x=72 \mathrm{~km}$, from which $x=6$ hours.