✔ 最佳答案
_____________ A ─────────────────── B
_____________ |------------------100km ------------------|
............<<< David(15 km/h)..........................<<< Alice(y km/h)
a) After 2 hours , David is 2(15) + 100 km to the west of B , while
Alice is 2y km to the west of B.∴ The distance between David and Alice after 2 hours
= 2(15) + 100 - 2y
= 130 - 2y km
b) After 2.5 hours , David is 2.5(15) + 100 km to the west of B , while
Alice is 2.5y km to the west of B.Alice catched up with David means that
2.5(15) + 100 = 2.5y
137.5 = 2.5y
y = 55
2012-12-04 19:17:05 補充：
2(15) to the west of A.
2(15) + 100 to the west of B.
