Difference between two rate questions!

Expert replies

Codebreaker2k: Junior | Next Rank: 30 Posts; Posts: 12; Joined: Sun Nov 25, 2012 8:01 pm

by Codebreaker2k » Tue Feb 05, 2013 4:43 pm

Question 1:

If Eileen drives at an average speed of 50 miles per hour for 3 hours and an average speed of 60 miles per hour for the next 2 hours, what is her average speed for the entire trip?

Answer: 3(50)=150, 60(2)=120 so (150+120)/5=54

Question 2:

A train travels at a speed of 60 mph for the first 180 miles of its trip, then it travels at a speed of 45 mph for the remaining 180 miles of its trip. What is the train's average speed for the entire trip?

Answer: 180/60=3, 180/45=4 so (180+180)/(4+3)=51 and 3/7.

MY QUESTION: In the first question we used multiplication initially, and in the second we used division first. How does one differentiate when to multiply or divide initially? I want to make sure I am translating the question correctly.

Thank you!

Like this post

Source: Beat The GMAT — Problem Solving | Tue Feb 05, 2013 4:43 pm

vkb001: Junior | Next Rank: 30 Posts; Posts: 23; Joined: Wed Feb 23, 2011 7:32 amFollowed by: 2 members; Thanked: 8 times; GMAT Score:740

by vkb001 » Tue Feb 05, 2013 4:56 pm

a. Average speed is total distance travelled, divided by total time taken to travel that total distance.
b. Distance = Speed * Time

In Question 1:
To find average speed, you have total time given. You need total distance travelled. Therefore, you use formula (b), where you MULTIPLY speed and time, to get the distance and formula (a) to find average speed.

In Question 2:
Here, you have speed and distance given. You need total time. To get time, you need to DIVIDE distance by speed. And given this new information, calculate average speed.

Some unsolicited advice: I'd suggest you remember the concept and not when to multiply and divide. That way, even if there's a curved-ball thrown at you, you'd be able to answer it easily in the exam.

Like this post

GMAT/MBA Expert

ceilidh.erickson: GMAT Instructor; Posts: 2095; Joined: Tue Dec 04, 2012 3:22 pmFollowed by: 247 members; Thanked: 1443 times

by ceilidh.erickson » Thu Feb 07, 2013 11:48 am

vkb001 is absolutely right! Don't try to memorize which situations require multiplication and which require division. Think about the relationship between rate (speed), time, and distance: R*T=D. I recommend writing down that formula whenever you see a rate problem, then ask yourself - which pieces do I have?

One simple way to keep this all organized is to use a rate chart. This will make it easy to see when you multiply and when you divide. For the first question:

The chart can help you see that what's missing is the distance for each leg of the journey. Multiply the rate by the time to find these. Average speed = (total distance)/(total time), so calculate: average speed = 54.

Second problem:

For the second problem, the chart helps you to see that the missing information is the time. Divide each distance by the respective rate to find the time. The add up your total distance, your total time, and divide: average speed = 360/7.

Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review