an impossibility in itself (that if there were TWO of her walking up and down, etc.) because it is impossible for her to be in two places at the same time.
I get the answer (that they will "meet") but in reality, they would never meet because there's only one of her and she can only exist in one place at one time. The "if it were the same day and one was going up and one was going down" isn't possible, so it could never happen anyways.
😉