Person X travels from A to B and then back the same route at a constant speed of 50 km/h. Person Y travels the same route to B at half the speed. How fast does person Y have to travel back to make the complete return trip in the same time as person X?
Let V be the speed of person X (50 km/h) and D be the distance travelled, one-way. Let the unkvown return speed of person Y be V'.
2D/V = D/(V/2) + D/V'
D/V' = 0
V' = infinite
By the time Y reaches the turnaround point, X has already completed the round trip. Y cannot catch up.