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# Tips and Hints for Specific Quant Topics (with Examples)

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Math Expert
Joined: 02 Sep 2009
Posts: 32657
Followers: 5659

Kudos [?]: 68754 [15] , given: 9818

Tips and Hints for Specific Quant Topics (with Examples) [#permalink]

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03 Jun 2014, 06:01
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New project from GMAT Club

Tips and Hints for Specific Quant topics with Examples.
We will be posting 2 questions every week to illustrate common shortcuts and tips for solving quant questions faster and better - Join in and bookmark this topic or subscribe to it to get notifications when new topics and tips get posted.

Number Properties:
PS question
DS Question
Tips and hints

Divisibility/Multiples/Factors:
PS question
DS question
Tips and hints

Remainders:
PS question
DS question
Tips and hints

Inequalities:
PS question
DS question
Tips and hints

Absolute Values:
PS question
DS question
Tips and hints

Algebra:
PS question
DS question
Tips and hints

Exponents and roots:
PS question
DS question
Tips and hints

Combinations:
PS question
DS question

Every week we'll be publishing 2 brand new (PS an DS) questions from each and every topic tested on the GMAT along with respective tips and hints.
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Last edited by Bunuel on 26 Jul 2014, 16:13, edited 37 times in total.
MBA Section Director
Affiliations: GMAT Club
Joined: 22 Feb 2012
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City: Pune
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Kudos [?]: 2199 [3] , given: 1756

Re: Tips and Hints for Specific Quant Topics (with Examples) [#permalink]

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26 Jul 2014, 11:23
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Here is a Tip on work/rate problems. I would like to introduce Percent approach to solve Work/rate problems.

Let's consider basic example.
A can complete a certain task in 10 hours and B can complete the same task 20 hours.
We assume that the task is 100%
So if A can complete the task in 10 hours, we can say he is taking 10 hours to complete 100% of task. That means in one hour he is finishing $$\frac{100%}{10}$$ or 10% task.
Similarly B is completing $$\frac{100%}{20}$$ or 5% task in one hour.
If they start working together(at their respective rates), they would finish (10% + 5%) or 15% task in one hour.
So to finish entire task together, it would take them $$\frac{100}{15}$$ or $$6\frac{2}{3}$$ hours.

Now lets apply this logic on following GMAT Type question. This is question is taken from Bunuel's Set of Mixed questions.

Three workers, A, B, and C, can complete a certain task in 10, 5 and x hours respectively. A starts working alone and 2 hours later B joins. After another 2 hours joins C. After that A, B, and C together complete the task in 15 minutes. What is the value of x?

A. 1
B. 1.25
C. 2
D. 2.5
E. 4

Three workers, A, B, and C, can complete a certain task in 10, 5 and x hours respectively
So in one hour A is completing 10% task, B is completing 20% task, and C is completing $$(\frac{100}{x})%$$ task.

A starts working alone and 2 hours later B joins.
Before B joins, A will have worked for 2 hours and have finished 20% task. That means when B joins A, total remaining work will be 80%

After another 2 hours joins C.
Using Similar logic, At the time of C's joining, A & B will have finished 60% task. Remaining work will be 20%

After that A, B, and C together complete the task in 15 minutes.
So A, B, C together complete remaining 20% task in 15 mins or in $$\frac{15}{60} hours$$

So we have that $$\frac{20%}{(10% + 20% + 100/x)} = \frac{15}{60}$$

Solving for X, we get 750x = 1500 ----------> x = 2

Hope that helps!!
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Math Expert
Joined: 02 Sep 2009
Posts: 32657
Followers: 5659

Kudos [?]: 68754 [0], given: 9818

Re: Tips and Hints for Specific Quant Topics (with Examples) [#permalink]

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26 Jul 2014, 16:13
Expert's post
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Re: Tips and Hints for Specific Quant Topics (with Examples)   [#permalink] 26 Jul 2014, 16:13
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