A man travels from point A to B at 40 km/h, further from B to Cat 50 km h and then further from to D at 60 km/h. The ratio between the distances AB, BC and CD is 1 : 2 : 3. He returns from D to A at x km/h. If his average speed for the entire journey is 480/13 km/h,  then what is the value of x?