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(1^2+2^2+3^3+…+20^2) / (1+2+3+ …+20)=?

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(1^2+2^2+3^3+…+20^2) / (1+2+3+ …+20)=?

by Max@Math Revolution » Thu Sep 13, 2018 12:32 am

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[Math Revolution GMAT math practice question]

( 1^2+2^2+3^2+...+20^2) / (1+2+3+ ...+20)=?

A. 41/3
B. 41/6
C. 41
D. 210
E. 420 $$$$
Last edited by Max@Math Revolution on Sun Sep 16, 2018 11:55 am, edited 1 time in total.
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by regor60 » Thu Sep 13, 2018 5:21 am
Max@Math Revolution wrote:[Math Revolution GMAT math practice question]

(1^2+2^2+3^3+...+20^2) / (1+2+3+ ...+20)=?

A. 41/3
B. 41/6
C. 41
D. 210
E. 420
It's not clear what you intend the numerator to be
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(1^2+2^2+3^3+…+20^2) / (1+2+3+ …+20)=?

by fskilnik@GMATH » Thu Sep 13, 2018 8:07 am
Max@Math Revolution wrote:[Math Revolution GMAT math practice question]

(1^2+2^2+3^2+...+20^2) / (1+2+3+ ...+20)=?

A. 41/3
B. 41/6
C. 41
D. 210
E. 420
The sum 1+2+3+...+20 is 20*(1+20)/2 = 21*10 and it is certainly in-the-GMAT-scope. (We have used a"finite arithmetic progression" important result.)

The sum of the squares of the first "n" (n=20) positive integers is out-of-GMAT´s scope, although a beautiful (and elementary) deduction for the formula:
\[{1^2} + {2^2} + \ldots + {n^2} = \frac{{n\left( {n + 1} \right)\left( {2n + 1} \right)}}{6}\]
you may find here: https://mathschallenge.net/library/numb ... of_squares

From the identity above, we have:
\[{1^2} + {2^2} + \ldots + {20^2} = \frac{{20 \cdot 21 \cdot 41}}{6} = 10 \cdot 7 \cdot 41\]

Finally,
\[? = \frac{{10 \cdot 7 \cdot 41}}{{21 \cdot 10}} = \frac{{41}}{3}\]

The right answer is therefore [spoiler]__(A)_____[/spoiler] .


This solution follows the notations and rationale taught in the GMATH method.

Regards,
Fabio.

P.S.: perhaps there is a smart way of avoiding the "out-of-scope formula"... some "hidden pattern", for instance...
Anyway, I could not find an alternate approach in approx. 5min (a generous"time-limit" in the GMAT)...
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by GMATGuruNY » Thu Sep 13, 2018 8:47 am
Max@Math Revolution wrote:[Math Revolution GMAT math practice question]

(1^2+2^2+3^2²+...+20^2) / (1+2+3+ ...+20)=?

A. 41/3
B. 41/6
C. 41
D. 210
E. 420
Since the answer choices are very spread out, we can BALLPARK -- and we can be somewhat lax with our estimations.

Numerator:
If the 11 greatest terms were 10²=100, the sum of the 11 greatest terms = 11*100 = 1100.
If the 11 greatest terms were 20²=400, the sum of the 11 greatest terms = 11*400 = 4400.
Implication:
The sum of the 11 greatest terms is around the average of 1100 and 4400:
(1100+4400)/2 = 2750.
The 9 smallest terms -- the perfect squares between 1 and 81, inclusive -- will increase the sum by another 200 or so.
Thus, the numerator ≈ 2750 + 200 ≈ 3000.

Denominator:
The denominator is composed of the integers between 1 and 20, inclusive.
For any evenly spaced set, sum = (number of terms)(average of first and last).
Thus, the denominator = 20 * (1+20)/2 = 210.

Approximate value of the given fraction:
3000/210 = a bit less than 15.
Only A is viable:
41/3 = 13â…”.

The correct answer is A.
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by fskilnik@GMATH » Thu Sep 13, 2018 9:07 am
GMATGuruNY wrote:Numerator:
If the 11 greatest terms were 10²=100, the sum of the 11 greatest terms = 11*100 = 1100.
If the 11 greatest terms were 20²=400, the sum of the 11 greatest terms = 11*400 = 4400.
Implication:
The sum of the 11 greatest terms is around the average of 1100 and 4400:
(1100+4400)/2 = 2750.
The 9 smallest terms -- the perfect squares between 1 and 81, inclusive -- will increase the sum by another 200 or so.
Thus, the numerator ≈ 2750 + 200 ≈ 3000.
Hi, Mitch! You had a creative idea, no doubt, but...

01. You have made CONSIDERABLE modifications in the original parcels... the alternative choices are not that apart for these modifications... far from that!
02. We are dealing with sum of SQUARES, arithmetic means (averages) are dangerous (to say the least) in nonlinear situations.

There was a "happy ending", I saw... but quoting the great Niels Bohr...

"Prediction is very difficult, especially about the future."

Regards,
Fabio.
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(1^2+2^2+3^3+…+20^2) / (1+2+3+ …+20)=?

by fskilnik@GMATH » Thu Sep 13, 2018 10:21 am
Max@Math Revolution wrote: (1^2+2^2+3^2+...+20^2) / (1+2+3+ ...+20)=?

A. 41/3
B. 41/6
C. 41
D. 210
E. 420
I guess I found a nice approximation solution inspired by Mitch´s clever idea.
(If he accepts the reasoning, we share the merits!)

\[{1^2} + {2^2} + \ldots + {20^2} < 10 \cdot {10^2} + 10 \cdot {20^2}\]
\[? = \frac{{{1^2} + {2^2} + \ldots + {{20}^2}}}{{10 \cdot 21}} < \frac{{{{10}^2} + {{20}^2}}}{{21}} = \frac{{500}}{{21}}\,\,\mathop \cong \limits^{\left( * \right)} \,\,24\]
\[\left( * \right)\,\,\,\frac{{500}}{{21}} = \frac{{420 + 84 - 4}}{{21}} = 23\frac{{17}}{{21}}\]

\[\left( A \right)\,\,\frac{{41}}{3} = 13\frac{2}{3}\]

Note that (C), (D) and (E) are out, while (B) is half the value of (A)... "much much less" (?) than 24...

Obs.: (first edited) we may divide the sum of squares into FOUR parcels (not TWO, as before) to MAJORATE and also MINORATE (second edited) our numerator-focus with better precision:
\[\frac{{5\left( {{3^2} + {8^2} + {{13}^2} + {{18}^2}} \right)}}{{10 \cdot 21}}\,\,\, < \,\,\,?\,\,\, < \,\,\,\frac{{5\left( {{5^2} + {{10}^2} + {{15}^2} + {{20}^2}} \right)}}{{10 \cdot 21}}\]
\[13\frac{{10}}{{21}}\,\, = \,\,\frac{{283}}{{21}}\,\,\, < \,\,\,?\,\,\, < \,\,\,\frac{{125}}{7}\,\, = \,\,17\frac{6}{7}\]

Now we are sure (A) is the right answer... and this COULD be done in less than 5min!

Regards,
Fabio.
Last edited by fskilnik@GMATH on Thu Sep 13, 2018 5:17 pm, edited 2 times in total.
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ballpark

by GMATGuruNY » Thu Sep 13, 2018 10:57 am
fskilnik wrote:
GMATGuruNY wrote:Numerator:
If the 11 greatest terms were 10²=100, the sum of the 11 greatest terms = 11*100 = 1100.
If the 11 greatest terms were 20²=400, the sum of the 11 greatest terms = 11*400 = 4400.
Implication:
The sum of the 11 greatest terms is around the average of 1100 and 4400:
(1100+4400)/2 = 2750.
The 9 smallest terms -- the perfect squares between 1 and 81, inclusive -- will increase the sum by another 200 or so.
Thus, the numerator ≈ 2750 + 200 ≈ 3000.
Hi, Mitch! You had a creative idea, no doubt, but...

01. You have made CONSIDERABLE modifications in the original parcels... the alternative choices are not that apart for these modifications... far from that!
02. We are dealing with sum of SQUARES, arithmetic means (averages) are dangerous (to say the least) in nonlinear situations.

There was a "happy ending", I saw... but quoting the great Niels Bohr...

"Prediction is very difficult, especially about the future."

Regards,
Fabio.
The answer choices represent the ratio between the numerator and the denominator of the given expression.
Option A implies that the numerator is about 14 times the denominator
Option B implies that the numerator is less than 7 times the denominator.
Option C implies that the numerator is 41 times the denominator.
Option D implies that the numerator is 210 times the denominator.
Option E implies that the numerator is 420 times the denominator.
The ratios implied by the answers choices are quite different.
The estimations in my earlier post are sufficient to determine whether the numerator is less than 7 times the denominator, about 14 times the denominator, 41 times the denominator, 210 times the denominator, or 420 times the denominator.

Consider the extremes:
If the 11 greatest terms in the numerator were all 10²=100, the numerator = 11(100) + (sum of the 9 smallest terms) ≈ 1100 + 200 = 1300.
In this case, numerator/denominator ≈ 1300/200 = 6.5.
If the 11 greatest terms in the numerator were all 20²=400, the numerator = 11(400) + (sum of the 9 smallest terms) ≈ 4400 + 200 = 4600.
In this case, numerator/denominator ≈ 4600/200 = 23.
Thus, the correct ratio must be between 6.5 and 23 -- more specifically, somewhere in the MIDDLE of this range.
Only option A is viable.
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edit

by Max@Math Revolution » Sun Sep 16, 2018 5:26 pm
=>

Now, 1^2 + 2^2 + 3^2 + ... + n^2 = n(n+1)(2n+1)/6 and 1 + 2 + 3 + ... + n = n(n+1)/2. So,
(1^2+2^2+3^2+...+20^2) / (1+2+3+ ...+20) = ( 20*21*41/6 ) / ( 20*21/2 )
= (41/6) / (1/2) = 41/3

Therefore, the answer is A.

Answer: A
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