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

Geometry

This topic has 1 expert reply and 2 member replies
vinay1983 GMAT Destroyer!
Joined
14 Aug 2013
Posted:
643 messages
Followed by:
6 members
Thanked:
48 times
Target GMAT Score:
650+
Geometry Post Wed Sep 18, 2013 7:15 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    If the length of an edge of cube X is twice the length of an edge of cube Y, what is the ratio of the volume of cube Y to the volume of cube X?

    (A) 1/2
    (B) 1/4
    (C) 1/6
    (D) 1/8
    (E) 1/27

    How can the length be considered as the volume of the cube?
    Volume of a cube is l*w*h!

    _________________
    You can, for example never foretell what any one man will do, but you can say with precision what an average number will be up to!

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    Post Wed Sep 18, 2013 7:21 am
    vinay1983 wrote:
    If the length of an edge of cube X is twice the length of an edge of cube Y, what is the ratio of the volume of cube Y to the volume of cube X?

    (A) 1/2
    (B) 1/4
    (C) 1/6
    (D) 1/8
    (E) 1/27

    How can the length be considered as the volume of the cube?
    Volume of a cube is l*w*h!
    The volume of a rectangular solid (aka box) = (length)(width)(height), but the volume of a cube = (length of one side)^3

    To solve this question, let's assign some nice dimensions to each cube.

    Let's say that cube Y has dimensions 1x1x1
    This means that cube X has dimensions 2x2x2 [since the length of an edge of cube X is twice the length of an edge of cube Y]

    The volume of cube Y = 1x1x1 = 1
    The volume of cube X = 2x2x2 = 8

    So, the ratio of the volume of cube Y to the volume of cube X = 1/8 = D\

    Cheers,
    Brent

    _________________
    Brent Hanneson – Founder of GMATPrepNow.com
    Use our video course along with Beat The GMAT's free 60-Day Study Guide

    Enter our contest to win a free course.

    Thanked by: vinay1983
    GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMAT’s FREE 60-Day Study Guide and reach your target score in 2 months!
    theCodeToGMAT GMAT Titan
    Joined
    14 Aug 2012
    Posted:
    1556 messages
    Followed by:
    34 members
    Thanked:
    448 times
    Target GMAT Score:
    750
    GMAT Score:
    650
    Post Wed Sep 18, 2013 9:02 am
    Volume of Cube is (Side)^3 and not L*B*H.

    Let Side of CUBE Y be y
    The Side of CUBE X is 2y

    Ratio of Volume of Cube y to Cube x = (y)^3/(2y)^3
    = 1/8

    Answer [D]

    vinay1983 wrote:
    If the length of an edge of cube X is twice the length of an edge of cube Y, what is the ratio of the volume of cube Y to the volume of cube X?

    (A) 1/2
    (B) 1/4
    (C) 1/6
    (D) 1/8
    (E) 1/27

    How can the length be considered as the volume of the cube?
    Volume of a cube is l*w*h!

    Post Wed Sep 18, 2013 10:51 am
    Hi vinay1983,

    Brent's approach, TESTing values, is straight-forward and easy to duplicate on Test Day; it's exactly how I would have done it.

    Here's the Number Property behind this question.

    This question asks us to double each side of a cube (the fact that it's a cube is irrelevant though, the solution would be the same even if it was any type of "box")

    Initial Volume = (side)(2nd side)(3rd side)

    Since we're doubling each side, we'd have

    New Volume = (double)(double)(double)(Initial Volume)

    When you "double" and "double again" and "double again", you end up with (2)(2)(2)(what you started with) = 8(what you started with).

    This is why the answer is D.

    GMAT assassins aren't born, they're made,
    Rich

    _________________
    Contact Rich at Rich.C@empowergmat.com

    Best Conversation Starters

    1 ziyuenlau 45 topics
    2 Mo2men 24 topics
    3 NandishSS 9 topics
    4 rsarashi 6 topics
    5 divsragha 5 topics
    See More Top Beat The GMAT Members...

    Most Active Experts

    1 image description GMATGuruNY

    The Princeton Review Teacher

    100 posts
    2 image description Brent@GMATPrepNow

    GMAT Prep Now Teacher

    90 posts
    3 image description DavidG@VeritasPrep

    Veritas Prep

    72 posts
    4 image description Rich.C@EMPOWERgma...

    EMPOWERgmat

    72 posts
    5 image description Jay@ManhattanReview

    Manhattan Review

    55 posts
    See More Top Beat The GMAT Experts