Events & Promotions
|
|
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized
for You
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Apr 2711:00 AM EDT
-12:00 PM EDT
Prefer video-based learning? The Target Test Prep OnDemand course is a one-of-a-kind video masterclass featuring 400 hours of lecture-style teaching by Scott Woodbury-Stewart, founder of Target Test Prep and one of the most accomplished GMAT instructors
Apr 2610:00 AM EDT
-11:00 AM EDT
The Target Test Prep course represents a quantum leap forward in GMAT preparation, a radical reinterpretation of the way that students should study. Try before you buy with a 5-day, full-access trial of the course for FREE!
Apr 2808:00 AM PDT
-11:00 AM PDT
Whether you are just beginning your MBA journey or fine-tuning your path to a 700+ score, GMAT Day is designed to give you a competitive edge.
11 Sep 2020, 22:00
Kudos
Bookmarks
If a car is travelling at 60 km/h from A to B and then 100km/h from B to A what is his average speed for the trip?
I understand i can use the harmonic mean to solve this. \(2/(\frac{1}{60}+\frac{1}{100})= 75km/h\)
but how do we use alligations to solve this? or the seesaw method?
I can calculate the ratio of the times if I was given 75, but i mean this is a weighted average question of sorts right? We should be able to use the technique to, but I can't figure out how.
I understand i can use the harmonic mean to solve this. \(2/(\frac{1}{60}+\frac{1}{100})= 75km/h\)
but how do we use alligations to solve this? or the seesaw method?
I can calculate the ratio of the times if I was given 75, but i mean this is a weighted average question of sorts right? We should be able to use the technique to, but I can't figure out how.
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block below for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
13 Sep 2020, 06:28
Kudos
Bookmarks
Everything you're saying is correct - average speed is a kind of weighted average, weighted by the time you spend at each speed. But I would not suggest trying to use weighted average methods for average speed problems, for two reasons. For one, you normally aren't given the information you'd like to have, if you were going to use weighted average methods. Usually you're given distance information, and what you really want is information about time. And second, sometimes people get confused about what is responsible for the weighting (it can be easy to think it's the ratio of the distances that matters, rather than the ratio of times), and if you don't use the correct weights, you'll get the wrong answer.
Here, the distances are equal. So the ratio of the times spent will be d/60 to d/100, or 1/60 to 1/100, and multiplying by 300 to get integers, will be 5 to 3. Since more time is spent at 60 mph, the answer will be closer to 60. Using alligation (if anyone reading this is unfamiliar with "alligation", this likely won't make sense), we have:
---60-------A--------------100---
and the ratio of the distances to the middle average must be 3 to 5, so the smaller distance on the left is 3/8 of the entire distance between 60 and 100, so is (3/8)(40) = 15, and A = 60 + 15 = 75.
I use alligation all the time, but I'd just solve this problem, and most other average speed problems, using an average speed table, since that turns out to be easier most of the time. This is, however, the only kind of weighted average problem that shows up on the GMAT where I would avoid alligation.
Here, the distances are equal. So the ratio of the times spent will be d/60 to d/100, or 1/60 to 1/100, and multiplying by 300 to get integers, will be 5 to 3. Since more time is spent at 60 mph, the answer will be closer to 60. Using alligation (if anyone reading this is unfamiliar with "alligation", this likely won't make sense), we have:
---60-------A--------------100---
and the ratio of the distances to the middle average must be 3 to 5, so the smaller distance on the left is 3/8 of the entire distance between 60 and 100, so is (3/8)(40) = 15, and A = 60 + 15 = 75.
I use alligation all the time, but I'd just solve this problem, and most other average speed problems, using an average speed table, since that turns out to be easier most of the time. This is, however, the only kind of weighted average problem that shows up on the GMAT where I would avoid alligation.
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
