Inactive Tutor answered 05/16/20
a) x= 23/48d
b) The particles after the 2nd collisions will have different speeds yet agin. Although Particle B will have to rebound off the wall once more and it's speed will decrease again. So now Particle A is 5.5 times faster and continually moving to the left where there is no wall, thus no rebound for A to go back and collided a third time.
1st time with a video answer hopefully this is helpful, PM me or comment for any more explanation.