Author 
Message 
TAGS:

Hide Tags

eGMAT Representative
Joined: 04 Jan 2015
Posts: 2063

Three copying machines A, B, and C, working together at their
[#permalink]
Show Tags
Updated on: 13 Aug 2018, 05:56
Question Stats:
85% (00:51) correct 15% (01:47) wrong based on 127 sessions
HideShow timer Statistics
Solve Time and Work Problems Efficiently using Efficiency Method!  Exercise Question #1Three copying machines A, B, and C, working together at their respective constant rates, can do a copying work in 2 hours. B and C, working together at their respective constant rates, can do the same copying job in 4 hours. How many hours would it take A, working alone at its constant rate, to do the same job? A. 2 hours B. 3 hours C. 4 hours D. 5 hours E. 6 hours
To solve question 2: Question 2To read the article: Solve Time and Work Problems Efficiently using Efficiency Method!
Official Answer and Stats are available only to registered users. Register/ Login.
_________________
Register for free sessions Number Properties  Algebra Quant Workshop
Success Stories Guillermo's Success Story  Carrie's Success Story
Ace GMAT quant Articles and Question to reach Q51  Question of the week
Must Read Articles Number Properties – Even Odd  LCM GCD  Statistics1  Statistics2 Word Problems – Percentage 1  Percentage 2  Time and Work 1  Time and Work 2  Time, Speed and Distance 1  Time, Speed and Distance 2 Advanced Topics Permutation and Combination 1  Permutation and Combination 2  Permutation and Combination 3  Probability Geometry Triangles 1  Triangles 2  Triangles 3  Common Mistakes in Geometry Algebra Wavy line  Inequalities Practice Questions Number Properties 1  Number Properties 2  Algebra 1  Geometry  Prime Numbers  Absolute value equations  Sets
 '4 out of Top 5' Instructors on gmatclub  70 point improvement guarantee  www.egmat.com



Board of Directors
Status: QA & VA Forum Moderator
Joined: 11 Jun 2011
Posts: 4096
Location: India
GPA: 3.5
WE: Business Development (Commercial Banking)

Re: Three copying machines A, B, and C, working together at their
[#permalink]
Show Tags
23 May 2018, 09:03
EgmatQuantExpert wrote: Three copying machines A, B, and C, working together at their respective constant rates, can do a copying work in 2 hours. B and C, working together at their respective constant rates, can do the same copying job in 4 hours. How many hours would it take A, working alone at its constant rate, to do the same job? A. 2 hours B. 3 hours C. 4 hours D. 5 hours E. 6 hours
Let the total work be 4 units... So, Efficiency of A + B + C = 2 unit/hr. Further Efficiency of B + C = 1 unit/hr So, Efficiency of A = 1 unit/Hr So, the time required by A to complete the work will be 4 Hours, answer must be (C)
_________________
Thanks and Regards
Abhishek....
PLEASE FOLLOW THE RULES FOR POSTING IN QA AND VA FORUM AND USE SEARCH FUNCTION BEFORE POSTING NEW QUESTIONS
How to use Search Function in GMAT Club  Rules for Posting in QA forum  Writing Mathematical Formulas Rules for Posting in VA forum  Request Expert's Reply ( VA Forum Only )



eGMAT Representative
Joined: 04 Jan 2015
Posts: 2063

Re: Three copying machines A, B, and C, working together at their
[#permalink]
Show Tags
Updated on: 26 May 2018, 13:11
Solution Given:In this question it is mentioned that • Three printing presses A, B, and C, working together at their respective constant rates, take 2 hours to do a certain printing job. • However, if only B and C are working, they can complete the same printing job in 4 hours. To find: • The number of days A alone will take to complete the job Approach and Working: As the job remains same, we can assume the total job to be the 12 units (multiple of the LCM of 2 and 4) • A, B, and C together take 2 hours to complete 12 units of work • Therefore, in 1 hour all of them will do \(\frac{12}{2}\) = 6 units of work • B and C together take 4 hours to complete 12 units of work • Therefore, in 1 hour they will do \(\frac{12}{4}\) = 3 units of work Now, in 1hour time, A, B, and C together do 6 units, out of which B and C do 3 units work • Therefore, in 1 hour, A will do (6 – 3) = 3 units of work So, working alone at this constant rate, A will take \(\frac{12}{3}\) days = 4 hours to complete the same job. Hence, the correct answer is option C. Answer: CImportant Observation • As mentioned in the article, when we assume the total job to be the LCM of the given number of hours, the calculation becomes much easy as we can avoid fractional calculations. However, in this case we have assumed the total job to be a multiple of the LCM, which shows not only the LCM but any multiple of LCM value can give you the same answer.
_________________
Register for free sessions Number Properties  Algebra Quant Workshop
Success Stories Guillermo's Success Story  Carrie's Success Story
Ace GMAT quant Articles and Question to reach Q51  Question of the week
Must Read Articles Number Properties – Even Odd  LCM GCD  Statistics1  Statistics2 Word Problems – Percentage 1  Percentage 2  Time and Work 1  Time and Work 2  Time, Speed and Distance 1  Time, Speed and Distance 2 Advanced Topics Permutation and Combination 1  Permutation and Combination 2  Permutation and Combination 3  Probability Geometry Triangles 1  Triangles 2  Triangles 3  Common Mistakes in Geometry Algebra Wavy line  Inequalities Practice Questions Number Properties 1  Number Properties 2  Algebra 1  Geometry  Prime Numbers  Absolute value equations  Sets
 '4 out of Top 5' Instructors on gmatclub  70 point improvement guarantee  www.egmat.com



Senior Manager
Joined: 12 Feb 2015
Posts: 487

Re: Three copying machines A, B, and C, working together at their
[#permalink]
Show Tags
26 May 2018, 11:48
The rate at which A,B & C work together is \(\frac{1}{2}\) [i.e. half of one work is done in one hour by A, B & C working together] The rate at which B & C work together is \(\frac{1}{4}\) [i.e. onefourth of one work is done in one hour by B & C working together] The rate at which A works is \(\frac{1}{2}\frac{1}{4} = \frac{1}{4}\) [i.e. onefourth of one work is done in one hour by Machine A alone] Therefore the time taken by A alone is 4 hours.
_________________
"Please hit +1 Kudos if you like this post"
_________________ Manish
"Only I can change my life. No one can do it for me"



Senior Manager
Joined: 12 Feb 2015
Posts: 487

Re: Three copying machines A, B, and C, working together at their
[#permalink]
Show Tags
26 May 2018, 12:52
LCM Method: A, B & C working together take 2 hours B & C working together take 4 hours A alone ? LCM of 2 & 4 is 4 therefore: A, B & C working together make 4/2, i.e. 2 units per hour B & C working together make 4/4, i.e. 1 units per hour There A alone will make 1 units per hour. To make 4 units, Machine A will take 4 hours at the rate of 1 units per hour
_________________
"Please hit +1 Kudos if you like this post"
_________________ Manish
"Only I can change my life. No one can do it for me"



Intern
Joined: 16 May 2017
Posts: 41
GPA: 3.8
WE: Medicine and Health (Health Care)

Re: Three copying machines A, B, and C, working together at their
[#permalink]
Show Tags
03 Oct 2018, 00:24
Rate of work of A,B & C together=1/2 (work/hr) Rate of work of B & C together=1/4 (work/hr) Rate of work of A=1/21/4= 1/4 (work/hr) So, time taken by A to finish job alone= 4 hrs
Ans(C)




Re: Three copying machines A, B, and C, working together at their &nbs
[#permalink]
03 Oct 2018, 00:24






