Observe closely the first 2 rows.
We need to sum these two and the sum of the rest of the rows will follow. the rows are positive and negative alternately.
Also, the numbers in rows 3, 5, and 7 are the multiples of the numbers in row 1. Sum row 1 and to find the sum of rows 3, 5, and 7, multiply the sum of row 1 which is 28 by the corresponding number (which happens to be equal to the row number for the positive rows in this case).
Sum row 3 = 28*2
Sum row 5 = 28*5
Sum row 7 = 28*7
Similarly sum row 2 = -56
Sum Row 4 = -56*2
Sum row 6 = -56*3
Adding all these you get 28
gmatprep - sum question
This topic has expert replies
Source: Beat The GMAT — Problem Solving |
-
cramya
- Legendary Member
- Posts: 2467
- Joined: Thu Aug 28, 2008 6:14 pm
- Thanked: 331 times
- Followed by:11 members
I think the sum is 112.
Each row is a sequence in which terms differ by a constant and are arranged in ascending order.
For example for row 1 the sum is = number of terms*mean(middle term here since th e median is the same as mean)
7*8 = 56
If u do it for all we get -336 and +448
The difference is 112. The answer should be 112 and not 28
Are u saying u picked 28 or the correct answer is 28?
Each row is a sequence in which terms differ by a constant and are arranged in ascending order.
For example for row 1 the sum is = number of terms*mean(middle term here since th e median is the same as mean)
7*8 = 56
If u do it for all we get -336 and +448
The difference is 112. The answer should be 112 and not 28
Are u saying u picked 28 or the correct answer is 28?
GMAT/MBA Expert
-
lunarpower
- GMAT Instructor
- Posts: 3380
- Joined: Mon Mar 03, 2008 1:20 am
- Thanked: 2256 times
- Followed by:1535 members
- GMAT Score:800
there's a whole bunch of ways to do this problem.
here are two.
(1) use COLUMNS (probably the easiest way - definitely the smallest numbers):
the first column adds to -1 - 1 - 1 + 7 = 4.
the second column is exactly twice the first column, so it's 2(4).
the third column is exactly 3 times the first column, so it's 3(4).
etc.
so the total of all the columns is 1(4) + 2(4) + 3(4) + ... 7(4)
= (1 + 2 + 3 + ... + 7)(4)
= 28 x 4
= 112
(2) use ROWS
the first row adds to 28.
the second row is -2 times the first row, so it's -2(28).
the third row is 3 times the first row, so it's +3(28).
etc.
so the total of all the rows is 28 - 2(28) + 3(28) - ... + 7(28)
= (1 - 2 + 3 - 4 + 5 - 6 + 7)(28)
= (4)(28)
= 112
done
mm hmm
oh yeah.
here are two.
(1) use COLUMNS (probably the easiest way - definitely the smallest numbers):
the first column adds to -1 - 1 - 1 + 7 = 4.
the second column is exactly twice the first column, so it's 2(4).
the third column is exactly 3 times the first column, so it's 3(4).
etc.
so the total of all the columns is 1(4) + 2(4) + 3(4) + ... 7(4)
= (1 + 2 + 3 + ... + 7)(4)
= 28 x 4
= 112
(2) use ROWS
the first row adds to 28.
the second row is -2 times the first row, so it's -2(28).
the third row is 3 times the first row, so it's +3(28).
etc.
so the total of all the rows is 28 - 2(28) + 3(28) - ... + 7(28)
= (1 - 2 + 3 - 4 + 5 - 6 + 7)(28)
= (4)(28)
= 112
done
mm hmm
oh yeah.
Ron has been teaching various standardized tests for 20 years.
--
Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
--
Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
--
Learn more about ron
--
Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi
--
Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.
Yves Saint-Laurent
--
Learn more about ron
