Susan drove an average speed of 30 miles per hour for the first 60 miles of a tripthen at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip  Pick.
A) 35 B) 36 C) 45 D) 50 E) 55

Rationale: "avg . speed = total distance / total time total distance = 90 miles total time = 60 / 30 + 30 / 60 = 5 / 2 avg . speed = 36 answer - b"
Answer: B