A Motorist Makes a Trip of 180 Miles

Multiple Choice

A motorist makes a trip of 180 miles.For the first 90 miles she drives at a constant speed of 30 mph.At what constant speed must she drive the remaining distance if her average speed for the total trip is to be 40 mph?

A) 45 mph
B) 50 mph
C) 52.5 mph
D) 55 mph
E) 60 mph