Combined Work Problem

This topic has 2 expert replies and 0 member replies
JScherman Just gettin' started! Default Avatar
Joined
22 Feb 2014
Posted:
3 messages
Combined Work Problem Post Thu May 08, 2014 9:23 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Can't seem to figure this one out....

    Each of 10 machines works at the same constant rate doing a certain job. The amount of the time needed by the 10 machines working together, to complete the job is 16 hours. How many hours would be needed if only 8 of the machines, working together, were used to complete the job?

    A. 18
    B. 20
    C. 22
    D. 24
    E. 26

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    Post Thu May 08, 2014 9:34 am
    JScherman wrote:
    .

    Each of 10 machines works at the same constant rate doing a certain job. The amount of the time needed by the 10 machines working together, to complete the job is 16 hours. How many hours would be needed if only 8 of the machines, working together, were used to complete the job?

    A. 18
    B. 20
    C. 22
    D. 24
    E. 26
    The amount of the time needed by the 10 machines working together, to complete the job is 16 hours.
    So, we can say that the job requires a total of 160 MACHINE HOURS (10 machines each working 16 hours = 160 machine hours)

    How many hours would be needed if only 8 of the machines, working together, were used to complete the job?
    We need to divide the work of 160 machine hours among the 8 machines.
    160/8 = 20 hours

    Answer: B

    Cheers,
    Brent

    Here's a similar question to try: http://www.beatthegmat.com/work-problem-t275195.html

    _________________
    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.

    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!
    Post Thu May 08, 2014 11:55 am
    JScherman wrote:
    Can't seem to figure this one out....

    Each of 10 machines works at the same constant rate doing a certain job. The amount of the time needed by the 10 machines working together, to complete the job is 16 hours. How many hours would be needed if only 8 of the machines, working together, were used to complete the job?

    A. 18
    B. 20
    C. 22
    D. 24
    E. 26
    PLUG IN approach:

    Let the rate per machine = 1 unit per hour.
    Rate for 10 machines = 10 units per hour.
    In 16 hours, the amount of work produced by 10 machines = r*t = 10*16 = 160 units.
    To produce 160 units, the time required by 8 machines = w/t = 160/8 = 20 hours.

    The correct answer is B.

    INVERSE PROPORTION approach:

    The number of machines and the number of hours are INVERSELY PROPORTIONAL.
    As the number of machines DECREASES, the number of hours must INCREASE, so that the same amount of work is produced in each case.
    Thus, we can set up the following equation:
    (number of machines)(number of hours) = (number of machines)(number of hours)

    Since 10 machines require 16 hours, and we must determine how many hours will be required by 8 machines, we get:
    10*16 = 8x
    x = 20.

    _________________
    Mitch Hunt
    GMAT Private Tutor
    GMATGuruNY@gmail.com
    If you find one of my posts helpful, please take a moment to click on the "Thank" icon.
    Available for tutoring in NYC and long-distance.
    For more information, please email me at GMATGuruNY@gmail.com.

    Thanked by: feedrom
    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.

    Best Conversation Starters

    1 hazelnut01 62 topics
    2 NandishSS 20 topics
    3 rsarashi 16 topics
    4 GMATinsight 15 topics
    5 richachampion 14 topics
    See More Top Beat The GMAT Members...

    Most Active Experts

    1 image description GMATGuruNY

    The Princeton Review Teacher

    112 posts
    2 image description Matt@VeritasPrep

    Veritas Prep

    109 posts
    3 image description Rich.C@EMPOWERgma...

    EMPOWERgmat

    96 posts
    4 image description Brent@GMATPrepNow

    GMAT Prep Now Teacher

    87 posts
    5 image description DavidG@VeritasPrep

    Veritas Prep

    64 posts
    See More Top Beat The GMAT Experts