Welcome! Check out our free B-School Guides to learn how you compare with other applicants.
Login or Register
 

Average

This topic has 2 expert replies and 6 member replies
Bookmark
Previous Topic Next Topic
GmatKiss GMAT Titan Default Avatar
Joined
26 Jul 2011
Posted:
2790 messages
Followed by:
41 members
Thanked:
205 times
Target GMAT Score:
700+
GMAT Score:
640
Average Post Sun Aug 14, 2011 2:30 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    A certain car averages 25 miles per gallon of gasoline when driven in the city and 40 miles per gallon when driven on the highway. According to these rates, which of the following is closest to the number of miles per gallon that the car averages when it is driven 10 miles in the city and then 50 miles on the highway?

    - 28
    - 30
    - 33
    - 36
    - 38

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    sumgb Rising GMAT Star
    Joined
    24 Jul 2011
    Posted:
    64 messages
    Thanked:
    13 times
    Target GMAT Score:
    720+
    GMAT Score:
    610
    Post Sun Aug 14, 2011 3:33 pm
    we need to find the value of (total miles / total gallons of fuel used)

    if car runs of 25 miles on 1 gallon fuel in city, then for 10 miles in city car needs 10/25 = 2/5 gallons of fuel

    if car run for 40 miles on 1 gallon fuel on highway, then for 50 miles on highway car needs 50/40 = 5/4 gallons of fuel

    so (total miles / total gallon) = 60 / (2/5 + 5/4) = 400 / 11 = 36.33 (approx.)

    so I would go with answer choice D which is 36

    What is OA?

    Hope this helps...

    GMAT/MBA Expert

    GMATGuruNY GMAT Instructor
    Joined
    25 May 2010
    Posted:
    6111 messages
    Followed by:
    1043 members
    Thanked:
    5278 times
    GMAT Score:
    790
    Most Active Expert Most Responsive Expert Most Thanked Expert
    Post Mon Aug 15, 2011 2:21 am
    GmatKiss wrote:
    A certain car averages 25 miles per gallon of gasoline when driven in the city and 40 miles per gallon when driven on the highway. According to these rates, which of the following is closest to the number of miles per gallon that the car averages when it is driven 10 miles in the city and then 50 miles on the highway?

    - 28
    - 30
    - 33
    - 36
    - 38
    City miles:Highway miles = 10:50 = 1:5.
    Any values that conform to this ratio will yield the same average miles per gallon.

    Let city miles = 200 and highway miles = 1000. (200:1000 = 1:5.)
    Gas used in the city = 200/25 = 8.
    Gas used on the highway = 1000/40 = 25.
    Total gas used = 8+25 = 33.
    Total Miles/Total Gas = 1200/33 = 400/11 ≈ 36.

    _________________
    Mitch Hunt
    GMAT Private Tutor and Instructor
    GMATGuruNY@gmail.com
    If you find one of my posts helpful, please take a moment to click on the "Thank" icon.
    Contact me about long distance tutoring!

    Thanked by: GmatKiss
    Free GMAT Practice Test How can you improve your test score if you don't know your baseline score? Take a free online practice exam. Get started on achieving your dream score today! Sign up now.
    Fighting with GMAT Just gettin' started! Default Avatar
    Joined
    15 Aug 2011
    Posted:
    1 messages
    Post Mon Aug 15, 2011 8:01 am
    Why can't we use the weighted average to solve this question ?

    Weighted Average miles / gallon
    = [(25x1)+(40x5)]/6
    = 225/6
    = 37.5
    = 38 miles per gallon (Answer choice # E)

    GmatKiss GMAT Titan Default Avatar
    Joined
    26 Jul 2011
    Posted:
    2790 messages
    Followed by:
    41 members
    Thanked:
    205 times
    Target GMAT Score:
    700+
    GMAT Score:
    640
    Post Mon Aug 15, 2011 9:36 pm
    OA: D
    I have the same doubt! Why can't we use weighted average in this case?
    Experts, pls help!

    GMAT/MBA Expert

    GMATGuruNY GMAT Instructor
    Joined
    25 May 2010
    Posted:
    6111 messages
    Followed by:
    1043 members
    Thanked:
    5278 times
    GMAT Score:
    790
    Most Active Expert Most Responsive Expert Most Thanked Expert
    Post Tue Aug 16, 2011 5:23 am
    GmatKiss wrote:
    OA: D
    I have the same doubt! Why can't we use weighted average in this case?
    Experts, pls help!
    Imagine the problem said this:

    Quote:
    A certain car averages 25 miles PER HOUR when driven in the city and 40 miles PER HOUR when driven on the highway. According to these rates, which of the following is closest to the AVERAGE SPEED FOR THE ENTIRE TRIP when the car is driven 10 miles in the city and then 50 miles on the highway?

    - 28
    - 30
    - 33
    - 36
    - 38
    The problem above asks for the average speed for the entire trip.
    To calculate the average speed for the entire trip, we need to determine how much weight must be given to each rate (25 miles per hour and 40 miles per hour).
    The amount of weight given to each rate is the amount of TIME spent traveling at each rate.
    Thus, we calculate the times, then divide the total distance by the total time.

    The same holds true for the gasoline problem, which asks for the average number of miles PER GALLON.
    This is no different from asking for the average number of miles PER HOUR -- the average speed -- for the entire trip.
    To calculate the average number of miles per gallon for the entire trip, we need to determine how much weight must be given to each miles-per-gallon rate (25 miles per gallon and 40 miles per gallon).
    The amount of weight given to each miles-per-gallon rate is the amount of GASOLINE used when traveling at each rate.
    Thus, we calculate the amounts of gasoline used, then divide the total distance by the total amount of gasoline.

    _________________
    Mitch Hunt
    GMAT Private Tutor and Instructor
    GMATGuruNY@gmail.com
    If you find one of my posts helpful, please take a moment to click on the "Thank" icon.
    Contact me about long distance tutoring!

    Thanked by: Fighting with GMAT
    Free GMAT Practice Test How can you improve your test score if you don't know your baseline score? Take a free online practice exam. Get started on achieving your dream score today! Sign up now.
    sl750 GMAT Destroyer!
    Joined
    07 Jun 2011
    Posted:
    497 messages
    Followed by:
    1 members
    Thanked:
    37 times
    Post Tue Aug 16, 2011 6:29 am
    The bottom line is, in speed-distance, or work-rate problems, you have to make sure the relationship is correct.i.e speed= distance/time or rate = work/time. So multiplying 25 with 1 is wrong as that is like multiplying speed times distance

    Thanked by: Fighting with GMAT
    gmatboost GMAT Destroyer!
    Joined
    02 Aug 2011
    Posted:
    312 messages
    Followed by:
    32 members
    Thanked:
    128 times
    Test Date:
    June, 2006
    GMAT Score:
    780
    Post Wed Aug 17, 2011 2:34 pm
    To be specific, the quantities in this question are miles, gallons, and miles/gallon.
    The "RTD" equation that relates these is (Miles/Gallon) * Gallons = Miles

    The question asks for overall Miles/Gallon, so you need to find total gallons and total miles. You know total miles, so you need to figure out how to find total gallons.

    Let the equation be your guide:
    (Miles/Gallon) * Gallons = Miles
    Gallons = Miles / (Miles/Gallon)

    Multiplying 25 * 10 is multiplying Miles/Gallon * Miles, which (as you can see with the equation above) isn't the right calculation.

    The takeaway, in my opinion, is to always use units when you're in an unfamiliar situation.

    _________________
    Greg Michnikov, Founder of GMAT Boost

    GMAT Boost offers 250+ challenging GMAT Math practice questions, each with a thorough video explanation, and 100+ GMAT Math video tips, each 90 seconds or less.
    It's a total of 20+ hours of expert instruction for an introductory price of just $10.
    View sample questions and tips without signing up, or sign up now for full access.

    Also, check out the most useful GMAT Math blog on the internet here.

    jzw Rising GMAT Star Default Avatar
    Joined
    13 Feb 2012
    Posted:
    79 messages
    Followed by:
    2 members
    Thanked:
    2 times
    Test Date:
    3/16
    Target GMAT Score:
    760+
    Post Wed Mar 14, 2012 2:39 pm
    would anyone be willing to do the original problem using the rate pie method please? i just need to see it applied once and i should be able to re-apply it afterward. i really appreciate it. taking the gmat next week!

    Best Conversation Starters
    1 varun289 31 topics
    2 sana.noor 23 topics
    3 killerdrummer 21 topics
    4 Rudy414 19 topics
    5 sanaa.rizwan 14 topics
    See More Top Beat The GMAT Members...

    Most Active Experts
    1 image description Brent@GMATPrepNow

    GMAT Prep Now Teacher

    		 199 posts
    2 image description GMATGuruNY

    The Princeton Review Teacher

    		 134 posts
    3 image description Jim@StratusPrep

    Stratus Prep

    		 106 posts
    4 image description Anju@Gurome

    Gurome

    		 47 posts
    5 image description Jon@Admissionado

    Admissionado

    		 39 posts
    See More Top Beat The GMAT Experts