2004 AMC 12A Problem 15

Starting at opposite ends, when they first meet they have together run half the track. Between the first and second meetings, they together run a full track length. Since Brenda runs at a constant speed and covered $100$ meters before the first meeting, she covers $2 \cdot 100 = 200$ meters between the two meetings. Adding Sally’s $150$ meters over that same interval gives a track length of $200 + 150 = 350$ meters. Thus, the correct answer is C.