M7MBA wrote:In a 40-mile trip, the first 20 miles were traveled in 50mph. If the total trip average speed is 60mph, what should be the average speed in the last 20 miles?
A) 150
B) 75
C) 50
D) 45
E) 40
Here's one approach:
The total distance is 40 miles, and we want the average speed to be 60 miles per hour.
Average speed = (total distance)/(total time)
So, we get: 60 = (40 miles)/(total time)
Solve equation to get: total time =
2/3 hours
So, the TIME for the ENTIRE 40-mile trip needs to be
2/3 hours.
The driver completed the first 20 miles of a 40-mile trip at an average speed of 50 miles per hour.
How much time was spent on this FIRST PART of the trip?
time = distance/speed
So, time = 20/50 =
2/5 hours
The ENTIRE trip needs to be
2/3 hours, and the FIRST PART of the trip took
2/5 hours
2/3 hours -
2/5 hours =
10/15 hours -
6/15 hours
=
4/15 hours
So, the SECOND PART of the trip needs to take
4/15 hours
The SECOND PART of the trip is 20 miles, and the time is
4/15 hours
Speed = distance/time
So, speed = 20/(
4/15)
= (20)(15/4)
= 75 mph
Answer: B
Cheers,
Brent
