A car traveled from San Diego to San Francisco at an average speed of 63 miles per hour. If the journey back took twice as long, what was the average speed of the trip?
Choices:  A) 24 .  B) 32 .  C) 36 .  D) 42 .  E) 44 .

Rationale: "let the time taken be = x one way distance = 63 x total distance traveled = 2 * 63 x = 126 x total time taken = x + 2 x = 3 x average speed = 126 x / 3 x = 42 answer : d"
Answer: D