Any number between 550,000 and 649,999 will do that.
Line B is traveling faster. I know this because Line B has a steeper line than line A.
Since it is the midpoint we can say:
12x - 1 = 8x +11
All we have do to is get all the x's alone on one side
12x - 1 = 8x + 11
12x = 8x + 12 (add both sides by 1)
4x = 12 (subtract both sides by 8x)
x = 3 (divide both sides to get what x is)
I would say 504 basing it off of converting 2/3 into 4/6 and used sets of 84 to each be 1/6