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A can complete a project in 20 days and B can complete the

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A can complete a project in 20 days and B can complete the [#permalink] New post 21 Sep 2012, 05:16
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A can complete a project in 20 days and B can complete the same project in 30 days. If A and B start working on the project together and A quits 10 days before the project is completed, in how many days will the project be completed?

(A) 18 days
(B) 27 days
(C) 26.67 days
(D) 16 days
(E) 12 days
[Reveal] Spoiler: OA
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Re: A can complete a project in 20 days and B can complete the [#permalink] New post 21 Sep 2012, 05:35
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154238 wrote:
A can complete a project in 20 days and B can complete the same project in 30 days. If A and B start working on the project together and A quits 10 days before the project is completed, in how many days will the project be completed?

(A) 18 days
(B) 27 days
(C) 26.67 days
(D) 16 days
(E) 12 days


The rate of A \frac{1}{20} job per day;
The rate of B \frac{1}{30} job per day.

Say they need t days to complete the project.

According to the stem we have that B works for all t days and A works only for t-10 days, thus \frac{1}{20}*(t-10)+\frac{1}{30}*t=1 --> t=18days.

Answer: A.

Hope it's clear.
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Re: A can complete a project in 20 days and B can complete the [#permalink] New post 21 Sep 2012, 05:49
154238 wrote:
A can complete a project in 20 days and B can complete the same project in 30 days. If A and B start working on the project together and A quits 10 days before the project is completed, in how many days will the project be completed?

(A) 18 days
(B) 27 days
(C) 26.67 days
(D) 16 days
(E) 12 days



Assume to total work to be 60 Units ( LCM of 20 & 30)
A does 3 units per day ( =60/20)
B does 2 units per day ( =60/30)
B works for 10 days alone work done = 2*10 = 20 units
A and B work together for the rest of the time
work left = 40 units
A and b together to 5 units oer day ( 2+3 = 5)
days required to do 40 units = 40/5 = 8

Total = 10+8 = 18
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Re: A can complete a project in 20 days and B can complete the [#permalink] New post 21 Sep 2012, 06:00
I am getting 27.. please tell me where i am going wrong.

The rate of A 1/20 job per day;
The rate of B 1/30 job per day:

If both are working together: 1/20 + 1/30 = 1/12, which means A and B working together will complete the project in 12 days.

if A left 10 days before the completion means they worked for 2 days together and B worked for 10 days more, so,
2*1/12 = 1/6 work done together => 5/6th project is remaining which wud be done by B,

B completes the project in.....30 days
B completes 5/6th project in......30*5/6=25

So, total of 25+2 = 27 days.

Please correct me if i am going in wrong direction.
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Re: A can complete a project in 20 days and B can complete the [#permalink] New post 21 Sep 2012, 06:13
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154238 wrote:
I am getting 27.. please tell me where i am going wrong.

The rate of A 1/20 job per day;
The rate of B 1/30 job per day:

If both are working together: 1/20 + 1/30 = 1/12, which means A and B working together will complete the project in 12 days.

if A left 10 days before the completion means they worked for 2 days together and B worked for 10 days more, so,
2*1/12 = 1/6 work done together => 5/6th project is remaining which wud be done by B,

B completes the project in.....30 days
B completes 5/6th project in......30*5/6=25

So, total of 25+2 = 27 days.

Please correct me if i am going in wrong direction.


Yes, working together they can complete the job in 12 days, but if A leaves at some point, then they no longer will need 12 days, they will need more. So, you cannot say that they worked together for 12-10=2 days.

Consider another approach:

In 10 days that B worked alone 10*1/30=1/3 of the job was done and 2/3 of the job was left to be done by A and B together.

Rate*Time=Job Done --> (1/20+1/30)*Time=2/3 --> Time=8. So, they worked together for 8 days, which means that total time is 10+8=18 days.

Hope it helps.
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PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. NEW!!!

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set. NEW!!!


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Re: A can complete a project in 20 days and B can complete the [#permalink] New post 21 Sep 2012, 10:26
A = w/20
B = w/30

A+B = w/20+w/30

Rate*Time = Work
(A+B)* (t-10) = W (A left the job 10 days before the total day)

solving we get t=28 days

putting total days = y-10= 28-10 = 18 days

I think I am wrong somewhere...
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Re: A can complete a project in 20 days and B can complete the [#permalink] New post 14 Nov 2012, 07:08
d is the total days to complete the project.
A worked for d-10 days while b worked the whole d days.

Setup the Rate Equation like this (Rate of A) (d -10) + (Rate of B) (d) = 1

(\frac{1}{20}) (d-10) + (\frac{1}{30}) d = 1, 1 represents 100% of the project
3d -30 + 2d = 60 ==> 5d = 90 ==> d = 18 days

Answer: A
Re: A can complete a project in 20 days and B can complete the   [#permalink] 14 Nov 2012, 07:08
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