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# (4*12+4*22+4*32+…+4*102)/(3*1+3*2+…+3*10)=?

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
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GPA: 3.82

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02 May 2016, 20:16
Expert's post
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Difficulty:

35% (medium)

Question Stats:

73% (01:47) correct 27% (02:02) wrong based on 48 sessions

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$$\frac{(4*1^2+4*2^2+4*3^2+…+4*10^2)}{(3*1+3*2+…+3*10)}=?$$

A. 71/9
B. 79/9
C. 28/3
D. 93/9
E. 125/9

* A solution will be posted in two days.
[Reveal] Spoiler: OA

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Last edited by Bunuel on 02 May 2016, 21:44, edited 3 times in total.
Moved to PS forum and edited the tags.
Math Expert
Joined: 02 Aug 2009
Posts: 5663

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02 May 2016, 20:30
Expert's post
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This post was
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MathRevolution wrote:
(4*12+4*22+4*32+…+4*102)/(3*1+3*2+…+3*10)=?

A. 71/9
B. 79/9
C. 28/3
D. 93/9
E. 125/9

* A solution will be posted in two days.

Hi,
two points..
1) it is not a MIXTURE problem..
2) (4*12+4*22+4*32+…+4*102) is not likely to give you 11 on numerator..
You are missing out on some signs.. as 12 may be 1^2.. pl recheck

The Q is supposed to be --
(4*12+4*22+4*32+…+4*102)/(3*1+3*2+…+3*10)= $$\frac{(4*1^2+4*2^2+4*3^2+…+4*10^2)}{(3*1+3*2+…+3*10)}$$

$$\frac{4(1^2+2^2+...+10^2)}{3(1+2+3...+10)} = \frac{4*10*(10+1)(2*10+1)}{6}/ \frac{3*10*11}{2}$$

=> $$\frac{4*10*11*21*2}{3*10*11*6} = \frac{28}{3}$$
C..

Ofcourse you require to know that $$1^2+2^2+...n^2 = \frac{n(n+1)(2n+1)}{6}$$
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Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

BANGALORE/-

Intern
Joined: 05 Sep 2014
Posts: 2

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02 May 2016, 20:37
According to my understanding, the answer does not match with any of the options as per my solution which is as follows:-

4(12+22+32....+102)/3(1+2+3+....10).
Now, both the numerator and denominator form an A.P. The sum of an A.p. is = n/2(first Term + Last Term)
4(570)/3(55) = 152/11.

Please let me know if i have made any mistake in the solution...
Intern
Joined: 06 May 2015
Posts: 25

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02 May 2016, 21:21
Hello Chetan2u.
however hit the right option but is the sum can be solved as-
we know that 4/3 is common numerator & denominator & if we look the option only c is the multiple of 4/3.
so no calculation is required.
correct if approach is wrong .
Math Expert
Joined: 02 Aug 2009
Posts: 5663

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02 May 2016, 21:28
shravan2025 wrote:
Hello Chetan2u.
however hit the right option but is the sum can be solved as-
we know that 4/3 is common numerator & denominator & if we look the option only c is the multiple of 4/3.
so no calculation is required.
correct if approach is wrong .

Yes , there are many Qs which can be solved by this approach..
BUT be careful, we do not know what comes out of (1+2+3....+10) because if a multiple of 4 comes out of it the above 4 in numerator can get cancelled..
But a bit of THINKING--- that in 10 numbers 5 are ODD and 5 are EVEN--- the SUM 1+2+..+10 will be ODD..

Now you can USE your approach since the Denominator is ODD*ODD so the NUMERATOR has to be EVEN because of 4..
as Observed by you , ONLY C fits in..
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Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

BANGALORE/-

Math Revolution GMAT Instructor
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GPA: 3.82

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05 May 2016, 21:35
We know that 1^2+2^2+….+n^2=n(n+1)(2n+1)/6 and 1+2+….+n=n(n+1)/2. Then, from the question, the numerator becomes 4(1^2+2^2+..10^2)=4(10)(11)(21)/6 and the denominator becomes 3(1+2+….+10)=3(10)(11)/2. Hence, (4*1^2+4*2^2+4*3^2+…+4*10^2)/(3*1+3*2+…+3*10) =[4(10)(11)(21)/6]/[3(10)(11)/2]=28/3. Hence, the correct answer is C.
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29 Jan 2018, 03:41
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Re: (4*12+4*22+4*32+…+4*102)/(3*1+3*2+…+3*10)=?   [#permalink] 29 Jan 2018, 03:41
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