During a trip on an expressway, Don drove a total of x miles. His average speed on a certain 5-mile section of the expressway was 30 miles per hour, and his average speed for the remainder of the trip was 50 miles per hour. His travel time for the x-mile trip was what percent greater than it would have been if he had traveled at a constant rate of 60 miles per hour for the entire trip?	Pick:	A) 8.5%	B) 50%	C) x/12%	D) 400/x%	E) 500/x%
Say x = 5 miles (so no remainder of the trip).   Time to cover x = 5 miles at 30 miles per hour = (time) = (distance)/(rate) = 5/30 = 1/6 hours = 10 minutes.   Time to cover x = 5 miles at 60 miles per hour = (time) = (distance)/(rate) = 5/50 = 1/10 hours = 5 minutes.   (Or simply, half rate will result in doubling the time.)   So, we can see that the time to cover x = 5 miles at 30 miles per hour (10 minutes) is 100% greater than the time to cover x = 5 miles at 60 miles per hour (5 minutes).   Now, plug x = 5 miles into the answer choices to see which one yields 100%. Only answer E works.   Answer: 400/x%.
<result>D</result>
During a trip on an expressway, Don drove a total of x miles. His average speed on a certain 5-mile section of the expressway was 30 miles per hour, and his average speed for the remainder of the trip was 50 miles per hour. His travel time for the x-mile trip was what percent greater than it would have been if he had traveled at a constant rate of 60 miles per hour for the entire trip?