Two trains start simultaneously from opposite ends of a 175-km route and travel toward each other on parallel tracks. Train X, traveling at a constant rate, completes the 175-km trip in 4 hours. Train Y, travelling at a constant rate, completes the 175-km trip in 3 hours. How many kilometers had Train X traveled when it met Train Y? Choose one
A) 66
B) 69
C) 72
D) 75
E) 78

Rationale: if the two trains cover a total distance d , then train x travels ( 3 / 7 ) * d while train y travels ( 4 / 7 ) * d . if the trains travel 175 km to the meeting point , then train x travels ( 3 / 7 ) * 175 = 75 km . the answer is d .
Answer: D