A person travels one-third of his journey at a speed of 40 km/h, the next one-third at a speed of 30 km/h, and the remaining part at a speed of 15 km/h. If his average speed in the entire journey was x km/h, then how much time (in hours) would he take to cover a distance of 144 km at an average speed of x km/h?

• A. \(5\dfrac{1}{2}\)
• B. 8
• C. \(6\dfrac{1}{2}\)
• D. 6