In a 100-mile trip, the first 20 miles were completed at 40 mph. If the average speed for the whole trip was 50 mph, what was the average speed in the last 80 miles? Choices
A) 155/3 mph	B) 160/3 mph	C) 165/3 mph	D) 170/3 mph	E) 175/5 mph
The time to complete the first 20 miles was (20 miles / 40 mph) = (1/2) hours.   The time to complete the whole trip was (100 miles / 50 mph) = (2) hours.   The time to complete the last 80 miles was (2) - (1/2) = (3/2) hours.   The average speed in the last 80 miles was (80 miles / (3/2) hours) = 160/3 mph.   The answer is 160/3 mph.
<result>B</result>
In a 100-mile trip, the first 20 miles were completed at 40 mph. If the average speed for the whole trip was 50 mph, what was the average speed in the last 80 miles?