Then came the first encounter: 75/Va=X-75/Vb, Va/Vb=75/(X-75).

Second meeting: 2x-55x/va = x+55/VB = (2x-55)/(x+55).

75/(X-75)=(2X-55)/(X+55) can be obtained from the above two formulas.

Ball = 75 or 0.

So X= 140km.