TOM traveled the entire 60 miles trip. If he did the first 12 miles of at a constant rate 24 miles per hour and the remaining trip of at a constant rate 48 miles per hour, what is the his average speed, in miles per hour?
Choose the correct choice from the following answers:
A) 20 mph	B) 24 mph	C) 30 mph	D) 40 mph	E) 42 mph

Rationale: "avg speed = total distance / total time = ( d 1 + d 2 ) / ( t 1 + t 2 ) = ( 12 + 48 ) / ( ( 12 / 24 ) + ( 48 / 48 ) ) = 60 * 2 / 3 = 40 mph d"
Answer: D