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# Three employees, A, B and C, clean a certain conference room each day.

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Intern
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Posts: 24
Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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26 Feb 2015, 10:36
4
28
00:00

Difficulty:

95% (hard)

Question Stats:

25% (02:43) correct 75% (02:45) wrong based on 499 sessions

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Three employees, A, B and C, clean a certain conference room each day. Working together, A and B can clean the conference room in 3 hours, whereas A and C together can do it in 2 and 1/2 hours. Can A, B and C working together clean the conference room in less than 2 hours?

1) B cleans faster than A.
2) Working alone, C can clean the conference room in less tahn 5 hours.
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Re: Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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27 Feb 2015, 23:40
9
4
Hi Awli,

The prompt is "layered" and more difficult than typical "Work" questions, but if you are comfortable converting the data into "work per hour" then you can get to the correct answer with a bit of effort. Knowing the 'Work Formula' will also help you when dealing with Fact 2.

We're given 2 rates for 2 groups of people:
1) A and B can clean the room in 3 hours
2) A and C can clean the room in 2.5 hours

We're asked if A, B and C can clean the room in UNDER 2 hours. This is a YES/NO question.

Fact 1: B cleans faster than A

Since A and B can clean the room in 3 hours, and we're told that B cleans FASTER than A, then that means that B does MORE than 1/2 of the work in that 3 hours:

> 1/2 the work in 3 hours =
> 3/6 of the work in 3 hours =
> 1/6 of the work in 1 hour

We now know that B can clean MORE than 1/6 of the room in 1 hour....

We also know that A and C can clean the room in 2.5 hours:

1/1 of the work in 2.5 hours =
2/5 of the work in 1 hour

NOW we know how much gets done per hour:
A and C get 2/5 of the room cleaned
B gets MORE than 1/6 of the room cleaned

2/5 + (more than 1/6) =
12/30 + (more than 5/30) =
MORE than 17/30 of the room is cleaned in 1 hour. By extension, 34/30 of the room would be cleaned in 2 hours (which is more cleaning than is necessary). This PROVES that the room will take LESS than 2 hours to clean and the answer to the question is ALWAYS YES.
Fact 1 is SUFFICIENT.

Fact 2: C can clean the room in LESS than 5 hours.

Here, we can use the Work Formula to our advantage....

Work = (X)(Y)/(X+Y) where X and Y are the times it takes to clean the room individually.

Here, we'll call....
X = the time for A and B to clean the room = 3 hours
Y = the time for C to clean the room = less than 5 hours

(3)(less than 5)/(3 + less than 5) = (less than 15)/(less than 8) = less than 15/8 = less than 2 hours and the answer to the question is ALWAYS YES.
Fact 2 is SUFFICIENT.

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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!***** ##### Most Helpful Community Reply Manager Joined: 02 May 2014 Posts: 100 Schools: ESADE '16, HKU'16, SMU '16 GMAT 1: 620 Q46 V30 Re: Three employees, A, B and C, clean a certain conference room each day. [#permalink] ### Show Tags 12 Mar 2015, 18:27 14 6 Awli wrote: Three employees, A, B and C, clean a certain conference room each day. Working together, A and B can clean the conference room in 3 hours, whereas A and C together can do it in 2 and 1/2 hours. Can A, B and C working together clean the conference room in less than 2 hours? 1) B cleans faster than A. 2) Working alone, C can clean the conference room in less than 5 hours. A + B , Time = 3 hrs, so per hr work = 33.33% of total work. A + C , Time = 2.5 hrs, so per hr work = 40% of total work. Need to know, whether A+B+C can complete in less than 2 hrs. or A+B+C can do more than 50% work per hr. 1) Rate of B faster than A. Therefore, B will do work which is greater than 1/2(33.33)% , i.e. greater than 16.5 % Now add B's effort in A+C's effort(40%). We get A+B+C's effort > 50 % Suff. 2) C can do more than 20% of the job per hr. Now add C's effort with A+B( 33.33%), i.e. A+B+C can do more than 50% of the work per hr. Suff. Hence (D) Please lemme know if there's anything wrong in my logic. Thanks!! ##### General Discussion Math Expert Joined: 02 Aug 2009 Posts: 7106 Re: Three employees, A, B and C, clean a certain conference room each day. [#permalink] ### Show Tags 27 Feb 2015, 03:54 8 3 Awli wrote: Three employees, A, B and C, clean a certain conference room each day. Working together, A and B can clean the conference room in 3 hours, whereas A and C together can do it in 2 and 1/2 hours. Can A, B and C working together clean the conference room in less than 2 hours? 1) B cleans faster than A. 2) Working alone, C can clean the conference room in less tahn 5 hours. hi Awli, ans D.. 1) statement one tells us B>A so both A and B do it in 3 hrs.. lets take the worst scenario that B is just above A or almost same so A's speed = B's speed = 6 hrs.. so C's speed = 2/5-1/6=7/30... so A,B and C can do it in 1/6+1/6+7/30=17/30.. so time taken =30/17=1 13/17 days<2 days... sufficient 2) let the worst scenario C takes 5 hrs so total time=1/3+1/5=8/15 or 15/8 days<2 days .. sufficient _________________ 1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372 2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html 3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html GMAT online Tutor EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 13074 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: Three employees, A, B and C, clean a certain conference room each day. [#permalink] ### Show Tags 12 Mar 2015, 21:02 1 Hi sytabish, Most Test Takers approach 'Work Formula' questions by converting data into a 'fraction of work done per hour' format or using the Work Formula (if it's two entities working on a job). Since data can be converted from one format to many different formats (fractions to decimals to percents to ratios, etc.), there is nothing wrong with what you did. It's a rather clever way to look at the data and just as valid as any other approach. 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 Co-Founder & GMAT Assassin Special Offer: Save$75 + GMAT Club Tests Free
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Re: Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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13 Mar 2015, 05:53
1
EMPOWERgmatRichC wrote:
Hi sytabish,

Most Test Takers approach 'Work Formula' questions by converting data into a 'fraction of work done per hour' format or using the Work Formula (if it's two entities working on a job). Since data can be converted from one format to many different formats (fractions to decimals to percents to ratios, etc.), there is nothing wrong with what you did. It's a rather clever way to look at the data and just as valid as any other approach.

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

Thanks for the confirmation EMPOWERgmatRichC !
I have been trying to use the percentage approach wherever I can for Work rate problems.
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Posts: 45
Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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Updated on: 29 Mar 2015, 00:19
6
Three employees, A, B and C, clean a certain conference room each day. Working together, A and B can clean the conference room in 3 hours, whereas A and C together can do it in 2 and 1/2 hours. Can A, B and C working together clean the conference room in less than 2 hours?

we have two equations :1) $$\frac{1}{A}+\frac{1}{B}=\frac{1}{3}$$

2) $$\frac{1}{A}+\frac{1}{C}=\frac{1}{2.5}$$

and we are asked , is $$\frac{1}{A}+\frac{1}{B}+\frac{1}{C}>\frac{1}{2}$$ ?

substitute equation 1.------->$$\frac{1}{3}+\frac{1}{C}>\frac{1}{2}$$--->6+2C>3C---->is 6>C ?.

Substitute equation 2.----->$$\frac{1}{2.5}+\frac{1}{B}>\frac{1}{2}$$--->5+2B>2.5B---> is 10>B ?.

check either of the above two conditions.

Statement 1): B cleans faster than A----> B<A , and from the equation 1($$\frac{1}{A}+\frac{1}{B}=\frac{1}{3}$$) if B cleans as fast as A then A=B=6 , But we are given that B<A then B must be less than 6 ----> B<6
Then condition, is 10>B ----> ALWAYS YES, because 10>6>B

Statement 1 SUFFICIENT

Statement 2): Working alone, C can clean the conference room in less tahn 5 hours.---> C<5
condition , is 6>C--->ALWAYS YES, because 6>5>C

Statement 2 SUFFIECIENT

ANS---> D

please correct me if i am wrong

Thanks,

Originally posted by AdlaT on 21 Mar 2015, 12:38.
Last edited by AdlaT on 29 Mar 2015, 00:19, edited 2 times in total.
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Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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28 Mar 2015, 10:33
Thank you very much to all of you. I've found very useful all the approaches.
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Re: Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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11 Jun 2016, 22:22
chetan2u wrote:
Awli wrote:
Three employees, A, B and C, clean a certain conference room each day. Working together, A and B can clean the conference room in 3 hours, whereas A and C together can do it in 2 and 1/2 hours. Can A, B and C working together clean the conference room in less than 2 hours?

1) B cleans faster than A.
2) Working alone, C can clean the conference room in less tahn 5 hours.

hi Awli,
ans D..
1) statement one tells us B>A so both A and B do it in 3 hrs.. lets take the worst scenario that B is just above A or almost same so A's speed = B's speed = 6 hrs..
so C's speed = 2/5-1/6=7/30... so A,B and C can do it in 1/6+1/6+7/30=17/30.. so time taken =30/17=1 13/17 days<2 days... sufficient
2) let the worst scenario C takes 5 hrs so total time=1/3+1/5=8/15 or 15/8 days<2 days .. sufficient

This is the best way to solve DS problems
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Re: Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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23 Sep 2016, 22:26
sytabish wrote:
Awli wrote:
Three employees, A, B and C, clean a certain conference room each day. Working together, A and B can clean the conference room in 3 hours, whereas A and C together can do it in 2 and 1/2 hours. Can A, B and C working together clean the conference room in less than 2 hours?

1) B cleans faster than A.
2) Working alone, C can clean the conference room in less than 5 hours.

A + B , Time = 3 hrs, so per hr work = 33.33% of total work.
A + C , Time = 2.5 hrs, so per hr work = 40% of total work.
Need to know, whether A+B+C can complete in less than 2 hrs. or A+B+C can do more than 50% work per hr.

1) Rate of B faster than A.
Therefore, B will do work which is greater than 1/2(33.33)% , i.e. greater than 16.5 %
Now add B's effort in A+C's effort(40%). We get A+B+C's effort > 50 %
Suff.

2) C can do more than 20% of the job per hr.
Now add C's effort with A+B( 33.33%), i.e. A+B+C can do more than 50% of the work per hr.
Suff.

Hence (D)

Please lemme know if there's anything wrong in my logic.

Thanks!!

Hey,
From where do you get that 1/2(33.33)% ?
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Re: Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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23 Feb 2017, 04:47
1
Prompt analysis
Let the rate of work done per hour for A, B, C be a,b,c respectively.According to the question:
(a+b)*3 = (b+c)*2.5 = (a+b+c)t

Superset
The answer could be YES or NO.

Translation
To find the value of t we need:
1# exact value of a, b, c.
2# some equations to find the value of t
3# any other information that will help to infer the value or range of t.

Statement analysis
St 1: b>a. lets take the worst scenario that B is just above A or almost same so A's speed = B's speed = 6 hrs..
so C's speed = 2/5-1/6=7/30... so A,B and C can do it in 1/6+1/6+7/30=17/30.. so time taken =30/17=1 13/17 days<2 days... sufficient
St 2: let the worst scenario C takes 5 hrs so total time=1/3+1/5=8/15 or 15/8 days<2 days .. sufficient
Option D
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Three employees, A, B and C, clean a certain conference room each day.  [#permalink]

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19 Dec 2017, 11:49
1
Let Rate of A = A, Rate of B = B, Rate of C = C

Given : A + B = 1/3 (since it takes 3 hours to clean together)
A + C = 2/5 (since it takes 5/2 hours to clean together)
Question: A + B + C > 1/2 ? (does it take less than 2 hours to clean together)
1/3 + C > 1/2 ?
C > 1/6 ? or does C take less than 6 hours to clean alone?

Statement 1: B cleans faster than A.
If B were to clean at same rate as A (worst case), A + B = 1/3 => A + A = 1/3 => A = 1/6
now we have A + C = 2/5 => C = 2/5 - 1/6 (worst case) = 7/30 (greater than 1/6)
but since B cleans faster than A, A < 1/6, so above eqn C = 2/5 - (<1/6) > (7/30) (again greater than 1/6)
=> Sufficient

Statement 2: Working alone, C can clean the conference room in less than 5 hours => directly answers our question -> suff

Kudos if you like my approach, need them badly to unlock my Gmat Club Tests

Thanks

Three employees, A, B and C, clean a certain conference room each day. &nbs [#permalink] 19 Dec 2017, 11:49
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# Three employees, A, B and C, clean a certain conference room each day.

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