Different samples

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Labenia: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Sat May 16, 2009 12:26 pm; Location: Germany

by Labenia » Sat May 16, 2009 11:55 pm

I've started my preparation for an exam and found out that I completely forgot maths (8 years ago is the last time I had to do with that).
I encountered this forum yesterday by chance (and I liked it at once

and here is my first posting in this community. I hope somebody can help me with understanding this simple task (the answer is known, but can't understand the logic behind it...).

If a quality control check is made by inspecting a sample of 2 lightbulbs from a box of 12 lightbulbs. How many different samples can be chosen?

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Source: Beat The GMAT — Problem Solving | Sat May 16, 2009 11:55 pm

Vemuri: Legendary Member; Posts: 682; Joined: Fri Jan 16, 2009 2:40 amFollowed by: 1 member; Thanked: 32 times

Re: Different samples

by Vemuri » Sun May 17, 2009 12:35 am

Hi, welcome to the forum.

The question is basically testing your knowledge on "combinations". Whenever you encounter the keywords "select" or "chose", think of combinations. The answer to the above question is 12c2 = 66ways

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Labenia: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Sat May 16, 2009 12:26 pm; Location: Germany

by Labenia » Sun May 17, 2009 1:57 am

Vemuri

OK. Thanks a bunch above all for your hint!

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Labenia: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Sat May 16, 2009 12:26 pm; Location: Germany

by Labenia » Sun May 17, 2009 3:09 am

Could someone please answer my general question, whether it's allowed to use a calculator in the GMAT exam?

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Vemuri: Legendary Member; Posts: 682; Joined: Fri Jan 16, 2009 2:40 amFollowed by: 1 member; Thanked: 32 times

by Vemuri » Sun May 17, 2009 3:21 am

Labenia wrote:Could someone please answer my general question, whether it's allowed to use a calculator in the GMAT exam?

Nope. Nothing is allowed in the GMAT exam. Not even your own pen. So, you can imagine....calculator is a distant allowance.

BTW....GMAT exam does not require the usage of calculator for any of the questions being tested.

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Labenia: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Sat May 16, 2009 12:26 pm; Location: Germany

by Labenia » Sun May 17, 2009 3:26 am

GMAT exam does not require the usage of calculator for any of the questions being tested.

Really? You mean tasks are usually pretty simple?
I wouldn't say so, though... I think my maths skills are quite poor that's why I need a calculator at least. Otherwise, it'll take for ages till I get a task calculated in mind

))))
But it's off topic.
Thanks again for your response.

May I bother you with one more question?

How should i know what is the root from 576800 without resorting to a calculator? Is there any hints on that?

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Vemuri: Legendary Member; Posts: 682; Joined: Fri Jan 16, 2009 2:40 amFollowed by: 1 member; Thanked: 32 times

by Vemuri » Sun May 17, 2009 4:00 am

Labenia wrote:GMAT exam does not require the usage of calculator for any of the questions being tested.

Really? You mean tasks are usually pretty simple?
I wouldn't say so, though... I think my maths skills are quite poor that's why I need a calculator at least. Otherwise, it'll take for ages till I get a task calculated in mind ))))
But it's off topic.
Thanks again for your response.

May I bother you with one more question?

How should i know what is the root from 576800 without resorting to a calculator? Is there any hints on that?

Well, I would imagine that the question would have a purpose for finding the root to 576800. The best way to approach such requirements is to find the prime factors of the number. For 576800, the factors are = (2^5)*(5^2)*7*103. Hope this helps.

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uroojk: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Sun May 31, 2009 1:10 pm

by uroojk » Sun May 31, 2009 1:12 pm

how would you do 12 c 2 without a calculator quickly?

i am having trouble figuring this out..
i guess it would help if i could figure out how to factorials without a calculator quickly

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lunarpower: GMAT Instructor; Posts: 3380; Joined: Mon Mar 03, 2008 1:20 amFollowed by: 1535 members; Thanked: 2256 times; GMAT Score:800

by lunarpower » Wed Jun 17, 2009 1:13 am

uroojk wrote:how would you do 12 c 2 without a calculator quickly?

i am having trouble figuring this out..
i guess it would help if i could figure out how to factorials without a calculator quickly

i'm going to assume that you can expand the expression "12 choose 2", since you mention it so casually.

this expression expands to (12!) / (10! 2!).

from what you're typing, it appears that you may actually be trying to evaluate this expression by multiplying out the factorials (!!!!) and then performing some sort of long division on the resulting numbers (!!!!).
this is really not the way to go.

instead, you should realize the following major takeaway:
larger factorials will always "swallow" smaller factorials.

here's what i mean: factorials consist of products of ALL positive integers up to and including the given integer. therefore, all of the integers that appear in some smaller factorial will also appear as part of a larger factorial.

so, in the quotient above, all of the numbers from 1 through 10 (from 10!) will also appear in 12!. you can thus cancel those numbers, leaving only the 11 and 12 remaining from 12!.
therefore, the fraction equals (11 x 12) / (1 x 2), or 11 x 6 = 66.

--

if you find yourself actually having to multiply out a factorial that is larger than, say, 5!, then you are probably missing out on LOTS of cancellation.
the gmat is not a test of "grinding arithmetic".

however, if you DON'T KNOW the shortcuts, then, by all means, you should try to grind out the computations. anything is better than sitting there staring at the problem.

Ron has been teaching various standardized tests for 20 years.

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