1. To go from X to Y, you either go forward 4 units, or go backward 4 units. To go from Y to Z, you either go forward 9 units, or go backward 9 units. So, to go from X to Y to Z, we have to either:
go backward 4 units and backward 9 units (total: backward 13)
go backward 4 units and forward 9 units (total: forward 5)
go forward 4 units and backward 9 units (total: backward 5)
go forward 4 units and forward 9 units (total: forward 13)
So, the distance from X to Z is either 5 or 13. Since 13 is not an answer choice, pick 5.
2. Use proportions: train speed = (s miles)/(t hours) = (y miles)/(x hours). Solve to get x = ty/s.
3. Since the experiment has only 3 disjoint outcomes, the probabilities must sum to 1. Thus, p + p/2 + p/4 = 7p/4 = 1, so p = 4/7.