Two vehicles A and B travel with uniform speed 30 km per hour and 60 km per hour respectively in the same direction. They start at the same time and from the same place for a distance of 120 km. The faster vehicle B reaches the destination and travels back with the same speed. Assume that the loss of time to change the direction is negligible. If x is the distance travelled by the slower vehicle A before the two vehicles cross each other, then what is the value of x?