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# http://www.manhattangmat.com. I am getting E. What is the

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Senior Manager
Joined: 05 May 2003
Posts: 427
Location: Aus
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Kudos [?]: 5 [0], given: 0

http://www.manhattangmat.com. I am getting E. What is the [#permalink]  09 Feb 2004, 07:20
http://www.manhattangmat.com.

I am getting E. What is the easy way to do these type problems ?? Thank you.
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SVP
Joined: 30 Oct 2003
Posts: 1797
Location: NewJersey USA
Followers: 5

Kudos [?]: 41 [0], given: 0

if the given function is true for all +ve N then it must be true for smallest possible N=1
We get
2!/1 * 4!/2! * 6!/2! .... 20!/10!

Each factor in the above product increases the power of 2 by 1 from the previous factor.

1+2+3+....10
This basically sum of 10 consecutive integers = n*(n+1)/2
10 * 11/2 = 55

Senior Manager
Joined: 23 Aug 2003
Posts: 462
Location: In the middle of nowhere
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How do I see the problem...when I click on the link Im going to Manhattan GMAT's homepage
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"Start By Doing What Is Necessary ,Then What Is Possible & Suddenly You Will Realise That You Are Doing The Impossible"

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Joined: 30 Oct 2003
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Kudos [?]: 41 [0], given: 0

Hi vivek,

I like u r avatar better now. Pervious one was too big and holding on to the one and only one achievement of our cricket team.

Here is the problem you requested. I think you need to register yourself. Each time you will login and then go to challenge problem of the week.

The sequence f(n) = (2n)! ├╖ n! is defined for all positive integer values of n. If x is defined as the product of the first 10 ten terms of this sequence, which of the following is the greatest factor of x?

(A) 2^20
(B) 2^30
(C) 2^45
(D) 2^52
(E) 2^55

Anand.
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Joined: 15 Dec 2003
Posts: 4312
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Hey Anandk, I got the same answer as you did (E). I started doing the calculations and got the answer only after seeing the pattern and obviously used n(n+1)/2 in order to find the exponent value. It is a long question though and sure took me more than 2 min.
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Best Regards,

Paul

Director
Joined: 03 Jul 2003
Posts: 655
Followers: 2

Kudos [?]: 26 [0], given: 0

Paul: How much time did you take to solve this?
Anand: How much time did you take to solve this?

I took little more than 5 minutes.
What is the best steagy to approach these kind of problems;
problems that can be solvable but takes a more than 4 mins to
solve?

Thanks
Bala.
GMAT Club Legend
Joined: 15 Dec 2003
Posts: 4312
Followers: 28

Kudos [?]: 219 [0], given: 0

took me about 4 minutes. I don't believe GMAT problems are this long. Problems like these are really there as an outstretch to make you think. The thinking process is good. As for this problem, once you see the pattern, you don't have to work out the whole thing.
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Best Regards,

Paul

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Joined: 30 Oct 2003
Posts: 1797
Location: NewJersey USA
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Kudos [?]: 41 [0], given: 0

it took me about 3 minutes but I did a stupid thing of calculating it in my head first. I knew I messedup in between because I was not getting any answer that matched. I started right but should have used pen and paper.
I used pen and paper finally and got it right.

Keep it in mind folks. Always use pen and paper right from the begining. I believe many people do it in their brain and get a wrong answer. Later they do it on paper and post the right answer. Getting it wrong here has no penalty. I feel one has to use pen and paper right from the begining. If one does not do it in this forum he/she will most likely will not do it in the test as well.
I did this and suffered many silly mistakes. Then my confidence went down and I was over cautious in further tests and endedup scoring less by locking horns on each and every silly problem. Had I used pen and paper right from the begining I would not have lost confidence and neither would I have made silly mistakes.
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