January 21, 2019 January 21, 2019 10:00 PM PST 11:00 PM PST Mark your calendars  All GMAT Club Tests are free and open January 21st for celebrate Martin Luther King Jr.'s Birthday. January 22, 2019 January 22, 2019 10:00 PM PST 11:00 PM PST In case you didn’t notice, we recently held the 1st ever GMAT game show and it was awesome! See who won a full GMAT course, and register to the next one.
Author 
Message 
TAGS:

Hide Tags

eGMAT Representative
Joined: 04 Jan 2015
Posts: 2457

R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
Updated on: 13 Aug 2018, 04:58
Question Stats:
42% (01:34) correct 58% (01:14) wrong based on 263 sessions
HideShow timer Statistics
Solve Time and Work Problems Efficiently using Efficiency Method!  Exercise Question #2R and S can complete a certain job in 6 and 4 days respectively, while they work individually. What will be the least number of days they will take to complete the same job, if they work on alternate days? A. 2.2 days B. 2.67 days C. 4.4 days D. 4.67 days E. 5 days
To solve question 3: Question 3To 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  Remainders1  Remainders2 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 PS Moderator
Joined: 26 Feb 2016
Posts: 3334
Location: India
GPA: 3.12

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
23 May 2018, 04:28
EgmatQuantExpert wrote: Solve Time and Work Problems Efficiently using Efficiency Method!  Exercise Question #2R and S can complete a certain job in 6 and 4 days respectively, while they work individually. What will be the least number of days they will take to complete the same job, if they work on alternate days? A. 2.2 days B. 2.67 days C. 4.4 days D. 4.67 days E. 5 days
Since R and S can complete a certain job in 6 and 4 days respectively, let's assume the total work done to be done as 24 units(A number that works with individual rates of 6 and 4). The individual rates are R  4 units/day and S  6 units/day. Working alternatively, R and S do 4+6+4+6 = 20 units of the work in 4 days. We are left with 4 units of work to do on the 5th day. On the fifth day, if R does the 4 units of work, it takes R 1 day to complete the work. if S does the 4 units of work, it takes S \(\frac{4}{6}\) or 0.67 day. The minimum amount of time is taken if S does the work on the fifth day. Since they work alternatively, S must have done work on the first and third day and R on the second and fourth day. Therefore, the total time taken for R and S to complete the work is 4.67 days(Option D)
_________________
You've got what it takes, but it will take everything you've got




Director
Joined: 11 Feb 2015
Posts: 678

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
26 May 2018, 10:51
LCM of 6 & 4 is 12 [Final answer does not change but this may be important for other questions as this amounts to a silly mistake] Also in your solution you have to specifically assume who is doing the work first? Since the question stem is asking the "least" time, then the person who works faster will start the work first. The order mentioned by you assumes RSRS (4+6+4+6 = 20). If this is the assumption then on the fifth day R will work again and he will take entire day to complete 4 units. Which means the answer in this case comes to (E) 4+1= 5 days, which is the wrong answer as the stem asks you to find out the "least time". (Your final answer is right, I am just clarifying the logic behind your answer because someone could ignore the order and commit a silly mistake as the trap answer in one of the options) Hence the correct order is S works on the first day because he does more units per day followed by R the next day. This gives us the following order of SRSR completing 20 units in first 4 days. On the 5th day S works for 2/3rd of the day as his individual rate is 6 units per day but he needs to complete only 4 units for which only 2/3 day or .67 day is required. Hence the answer is 4+.67=4.67 days. pushpitkc wrote: EgmatQuantExpert wrote: Solve Time and Work Problems Efficiently using Efficiency Method!  Exercise Question #2R and S can complete a certain job in 6 and 4 days respectively, while they work individually. What will be the least number of days they will take to complete the same job, if they work on alternate days? A. 2.2 days B. 2.67 days C. 4.4 days D. 4.67 days E. 5 days
Since R and S can complete a certain job in 6 and 4 days respectively, let's assume the total work done to be done as 24 units(LCM of 6 and 4). The individual rates are R  4 units/day and S  6 units/day. Working alternatively, R and S do 4+6+4+6 = 20 units of the work in 4 days. We are left with 4 units of work to do on the 5th day. On the fifth day, if R does the 4 units of work, it takes R 1 day to complete the work. if S does the 4 units of work, it takes S \(\frac{4}{6}\) or 0.67 day. The minimum amount of time is taken if S does the work on the fifth day. Therefore, the total time taken for R and S to complete the work is 4.67 days(Option D)
_________________
"Please hit +1 Kudos if you like this post"
_________________ Manish
"Only I can change my life. No one can do it for me"



Director
Joined: 11 Feb 2015
Posts: 678

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
26 May 2018, 11:13
Since R and S can complete a certain job in 6 and 4 days respectively, let's assume the total work done to be done as 12 units (LCM of 6 and 4). The individual rates are R  2 units/day and S  3 units/day. Since the question stem is asking the "least" time, then the person who works faster will start the work first. S works on the first day because he does more units per day followed by R the next day. This gives us the following order of SRSR completing 10 units in first 4 days. On the 5th day S works at his individual rate of 3 units per day but he needs to complete only 2 units for the entire work to be completed for which only 2/3 of the day or 0.67 day is required. Hence the answer is 4+.67=4.67 days. Correct Answer: Option D PS: Someone could ignore the order and commit a silly mistake as the trap answer is E which is one of the options.
_________________
"Please hit +1 Kudos if you like this post"
_________________ Manish
"Only I can change my life. No one can do it for me"



Senior PS Moderator
Joined: 26 Feb 2016
Posts: 3334
Location: India
GPA: 3.12

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
26 May 2018, 11:17
CAMANISHPARMAR wrote: Since R and S can complete a certain job in 6 and 4 days respectively, let's assume the total work done to be done as 12 units (LCM of 6 and 4).
The individual rates are R  2 units/day and S  3 units/day.
Since the question stem is asking the "least" time, then the person who works faster will start the work first. S works on the first day because he does more units per day followed by R the next day. This gives us the following order of SRSR completing 10 units in first 4 days. On the 5th day S works at his individual rate of 3 units per day but he needs to complete only 2 units for the entire work to be completed for which only 2/3 of the day or 0.67 day is required. Hence the answer is 4+.67=4.67 days.
Correct Answer: Option D
PS: Someone could ignore the order and commit a silly mistake as the trap answer is E which is one of the options. Thanks for noticing and notifying CAMANISHPARMARHave made the necessary changes in my solution.
_________________
You've got what it takes, but it will take everything you've got



eGMAT Representative
Joined: 04 Jan 2015
Posts: 2457

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
26 May 2018, 11:44
Solution Given:In this question, it is given that • R and S can complete a certain job in 6 days and 4 days respectively, when they work individually. To find: • We need to find out the least number of days they will take to complete the job, if they work on alternate days. Approach and Working: As R and S take 6 days and 4 days respectively while working individually, we can assume the total work to be LCM (6, 4) = 12 units • In 6 days, R can complete 12 units of work • Hence, in 1day R can complete \(\frac{12}{6}\) = 2 units of work • Similarly, in 4 days, S can complete 12 units of work • Hence, in 1day S can complete \(\frac{12}{4}\) = 3 units of work As they work on alternate days, the work that they will complete in a span of 2 days = (2 + 3) = 5 units • Therefore, in the first 4 days, they will complete 5 * 2 = 10 units of the total work Hence, remaining work = (12 – 10) units = 2 units Now to ensure that the completion of work happens in the least possible day, the person, who is more efficient among the 2, should work on the 5th day Among R and S, we know S is more efficient, and therefore, S should work on the 5th day to ensure the work gets completed at earliest • As S can complete 3 units of work in 1 day, to complete 2 units of work, S will take = \(\frac{2}{3}\) days = 0.67 days Hence, the least total number of days needed to complete the work = (4 + 0.67) days = 4.67 days Hence, the correct answer is option D. Answer: DImportant Observation • As they are working in alternate days, only one person is working in a day. • Hence, to ensure the job gets completed in least number of days, we need to ensure the more efficient person(S) works on the last day. • This also indicates that S should start the work on day 1, to ensure S works in the final day.
_________________
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  Remainders1  Remainders2 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



GMATH Teacher
Status: GMATH founder
Joined: 12 Oct 2010
Posts: 624

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
28 Sep 2018, 07:53
EgmatQuantExpert wrote: R and S can complete a certain job in 6 and 4 days respectively, while they work individually. What will be the least number of days they will take to complete the same job, if they work on alternate days? A. 2.2 days B. 2.67 days C. 4.4 days D. 4.67 days E. 5 days
Let´s imagine the job is defined by exactly 12 identical tasks (LCM(6,4) = 12). R does 2 tasks/day, while S does 3 tasks/day. FOCUS: minimize the number of days to do the 12 tasks... S is more efficient, let´s make HIM/HER start as soon as possible! (*) In four days, we have 3+2+3+2 = 10 tasks done. At the beginning of the 5th day, it is S who works (*) and using UNITS CONTROL, one of the most powerful tools of our course, we have: \(2\,\,\,{\rm{tasks}}\,\,\,\left( {{{1\,\,{\rm{day}}} \over {3\,\,{\rm{tasks}}}}\,\,\,\matrix{ \nearrow \cr \nearrow \cr } } \right)\,\,\,\,\, = \,\,\,{2 \over 3}\,\,{\rm{day}}\) Obs.: arrows indicate licit converter. \({\rm{?}}\,\,{\rm{ = }}\,\,{\rm{4}}{2 \over 3}\,\,{\rm{days}}\) This solution follows the notations and rationale taught in the GMATH method. Regards, Fabio.
_________________
Fabio Skilnik :: GMATH method creator (Math for the GMAT) Our highlevel "quant" preparation starts here: https://gmath.net



CEO
Joined: 11 Sep 2015
Posts: 3350
Location: Canada

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
28 Sep 2018, 08:27
EgmatQuantExpert wrote: R and S can complete a certain job in 6 and 4 days respectively, while they work individually. What will be the least number of days they will take to complete the same job, if they work on alternate days? A. 2.2 days B. 2.67 days C. 4.4 days D. 4.67 days E. 5 days
Let's assign a "nice" value to the job, a value that works well with the given values (6 days and 4 days ). So, let's say the ENTIRE job is to make 24 widgets R can complete a certain job in 6 daysIn other words, R can make 24 widgets in 6 days So, R can make 4 widgets per day S can complete a certain job in 4 daysIn other words, S can make 24 widgets in 4 days So, S can make 6 widgets per day What will be the least number of days they will take to complete the same job, if they work on alternate days?To MINIMIZE the time, the fastest worker (worker S) should go first. DAY 1: Worker S makes 6 widgets (running total of widgets made at the end of day 1 = 6) DAY 2: Worker R makes 4 widgets (running total of widgets made at the end of day 2 = 10) DAY 3: Worker S makes 6 widgets (running total of widgets made at the end of day 3 = 16) ASIDE: at this point, we can see that it will take more than 3 days to complete the job (ELIMINATE answer choices A and B)DAY 4: Worker R makes 4 widgets (running total of widgets made at the end of day 4 = 20) At this point, R and S have made 20 of the 24 needed widgets So, on day 5, worker S need only make 4 widgets. We already know that S can make 6 widgets per day, so it will take LESS THAN ONE day to make the remaining 4 widgets (ELIMINATE answer choice E) We can also conclude that, in 1/2 a day, S can make only 3 widgets. So, it will take MORE THAN 1/2 a day to make the remaining 4 widgets. (ELIMINATE answer choice C) Answer: D Cheers, Brent
_________________
Test confidently with gmatprepnow.com



Manager
Joined: 03 Mar 2017
Posts: 214

R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
28 Sep 2018, 22:33
The catch here is that you are not given the order. Although if any of them starts,after 2 days work completed will be 5/12. Similarly after another 2 days work done will be 5/12 again. Now we are left with(1/6) work. Here we have to decide the order. If we start with R on the first day, then it is his turn on the 5th day and he will take the whole day since his rate is 1/6. Since we are asked to find the least amount of time, we should start with S and therefore answer will be D. Very good question. Not so difficult but tricky.Read question carefully.
_________________
 All the Gods, All the Heavens, and All the Hells lie within you.



EMPOWERgmat Instructor
Status: GMAT Assassin/CoFounder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 13361
Location: United States (CA)

Re: R and S can complete a certain job in 6 and 4 days respectively, while
[#permalink]
Show Tags
24 Nov 2018, 16:22
Hi All, We're told that R and S can complete a certain job in 6 and 4 days respectively, while they work individually. We're asked for the LEAST number of days they will take to complete the same job, if they work on alternate days. Although the wording of this question is a bit 'clunky', the intent is that the machines will work on opposite days (for example, Machine R on the 1st day, Machine S on the 2nd day, Machine R on the 3rd day, etc.). This question can be approached in a number of different ways, including by determining what fraction of the job is completed each day. Machine R requires 6 days to complete a job, so it completes 1/6 of the job each day it works. Machine S requires 4 days to complete a job, so it completes 1/4 of the job each day it works. To complete this job in the fastest way possible, we should use the faster machine on Day 1... Day 1 Machine S > 1/4 of the job done = 6/24 done Day 2 Machine R > 1/6 of the job done = 4/24 done Day 3 Machine S > 1/4 of the job done = 6/24 done Day 4 Machine R > 1/6 of the job done = 4/24 done Etc. Every TWO days, 6/24 + 4/24 = 10/24 of the job is done After the 4th day, 20/24 is done. On the 5th day, Machine S is working. That machine will complete 6/24 of the job, but we only need 4/24 to complete the job. Thus, Machine S will need to work for only 2/3 of that last day.... Total work days = 4 + 2/3 = 4 2/3 Final Answer: GMAT assassins aren't born, they're made, Rich
_________________
760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com
Rich Cohen
CoFounder & GMAT Assassin
Special Offer: Save $75 + GMAT Club Tests Free
Official GMAT Exam Packs + 70 Pt. Improvement Guarantee www.empowergmat.com/
*****Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!*****




Re: R and S can complete a certain job in 6 and 4 days respectively, while &nbs
[#permalink]
24 Nov 2018, 16:22






