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In the first week of the Year, Nancy saved $1.  Question banks Downloads My Bookmarks Reviews Important topics Author Message TAGS: Manager Joined: 10 Aug 2009 Posts: 67 Location: United States (AL) Concentration: Entrepreneurship, Strategy GMAT 1: 600 Q43 V30 GMAT 2: 640 Q41 V36 WE: Business Development (Other) Followers: 0 Kudos [?]: 26 [3] , given: 45 In the first week of the Year, Nancy saved$1. [#permalink]  11 Nov 2012, 18:35
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In the first week of the Year, Nancy saved $1. In each of the next 51 weeks, she saved$1 more than she had saved in the previous week. What was the total amount that Nancy saved during the 52 weeks?

A. $1,326 B.$1,352
C. $1,378 D.$2,652
E. $2,756 [Reveal] Spoiler: OA _________________ "Popular opinion is the greatest lie in the world"-Thomas Carlyle Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 6062 Location: Pune, India Followers: 1596 Kudos [?]: 8930 [11] , given: 195 Re: In the first week of the Year, Nancy saved$1. [#permalink]  11 Nov 2012, 18:48
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Bigred2008 wrote:
In the first week of the Year, Nancy saved $1. In each of the next 51 weeks, she saved$1 more than she had saved in the previous week. What was the total amount that Nancy saved during the 52 weeks?

A. $1,326 B.$1,352
C. $1,378 D.$2,652
E. $2,756 Does anyone have a better way to do this? The total amount of money will be 1 + 2 + 3 + 4 + ... + 52 (in the 52nd week, she will save$52)

Sum of first n consecutive positive integers = n*(n+1)/2
Sum = 52*53/2 = 26*53
The product will end with 8 since 6*3 = 18 so answer must be (C)
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Hi,
This can be solved using the formula:

Sum= n/2( a + nd)

n=52

a= first term=1

d= difference =1

Sum=52/2(1+52)
=1378
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Re: In the first week of the Year, Nancy saved $1. [#permalink] 19 May 2013, 09:00 1 This post received KUDOS For an even list of consecutive numbers where a = the first number and z = the last number and (in this case) a total of 52 consecutive numbers-- (a+z)(.5*number of consecutive integers)---> (1+52)*26=1378 _________________ I do not beg for kudos. Intern Joined: 02 May 2013 Posts: 26 Concentration: International Business, Technology WE: Engineering (Aerospace and Defense) Followers: 1 Kudos [?]: 32 [0], given: 16 Re: In the first week of the Year, Nancy saved$1. [#permalink]  19 May 2013, 21:45
Simply using summation of first natural numbers formula = n(n+1)/2

=(52*53)/2 = 1378

Hence OA: C
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Re: In the first week of the Year, Nancy saved $1. [#permalink] 28 Oct 2013, 16:34 4 This post received KUDOS 1 This post was BOOKMARKED I used: Sum = (Average of terms) x (# terms) Average = (1+52) / 2 = 53/2 # terms= 52-1 + 1 = 52 Sum= 53*52/2 = 53*26 = 1378 Answer C. Intern Joined: 24 May 2014 Posts: 4 Followers: 0 Kudos [?]: 0 [0], given: 0 Re: In the first week of the Year, Nancy saved$1. [#permalink]  03 Jun 2014, 06:14
How did you know about that formula (n(n+1))/2 ? I've read through the GMAT Official Guide Math Review but don't recall learning that. It seems if you know that formula the question is very easy, but if you don't the problem can be a big time drain. Do you recommend other guides I should be reading to study up on tips and formulas for problems like these?
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Re: In the first week of the Year, Nancy saved $1. [#permalink] 03 Jun 2014, 20:08 1 This post received KUDOS Expert's post 1 This post was BOOKMARKED momentofzen wrote: How did you know about that formula (n(n+1))/2 ? I've read through the GMAT Official Guide Math Review but don't recall learning that. It seems if you know that formula the question is very easy, but if you don't the problem can be a big time drain. Do you recommend other guides I should be reading to study up on tips and formulas for problems like these? The sum of first n consecutive positive integers is given by the formula n(n+1)/2. For GMAT, it is a good idea to know this formula. It could simplify many calculations. Test prep companies discuss all such useful formulas in their curriculum. The Official Guides only give practice questions. You should also know the more generic formula of sum of an Arithmetic Progression. From that you can easily derive this formula. Sum of an Arithmetic Progression will be n*Average where n is the number of terms in the AP and Average will be the average value of the terms. The average value can be found as (First term + Last term)/2 If first term is a, last term is a + (n-1)*d where d is the common difference. Average = (a + a + (n-1)*d)/2 Sum = n*(a + a + (n-1)*d)/2 = n/2(2a + (n-1)*d) In case of consecutive integers starting from 1, a = 1 and d = 1 Sum = n/2(2 + (n-1)) = n(n+1)/2 Here are a couple of posts on APs: http://www.veritasprep.com/blog/2012/03 ... gressions/ http://www.veritasprep.com/blog/2012/03 ... gressions/ _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: In the first week of the Year, Nancy saved $1. [#permalink] 03 Jun 2014, 22:47 3 This post received KUDOS momentofzen wrote: How did you know about that formula (n(n+1))/2 ? I've read through the GMAT Official Guide Math Review but don't recall learning that. It seems if you know that formula the question is very easy, but if you don't the problem can be a big time drain. Do you recommend other guides I should be reading to study up on tips and formulas for problems like these? If you don't recall the formula, there is one method: 1 + 2 + 3 + 4 + 5 ........ + 26 + 27 + ............ + 48 + 50 + 51 + 52 Sum of 1st & last digit = 1 + 52 = 53 Sum of 2nd & 2nd last digit = 2 + 51 = 53 Sum of 3rd & 3rd last digit = 3 + 50 = 53 . . . . This is repeated 26 times So, 26 * 53 = 1378 Answer = C _________________ Kindly press "+1 Kudos" to appreciate Intern Joined: 21 May 2014 Posts: 12 Followers: 0 Kudos [?]: 2 [0], given: 24 Re: In the first week of the Year, Nancy saved$1. [#permalink]  04 Jun 2014, 11:08
PareshGmat wrote:
momentofzen wrote:
How did you know about that formula (n(n+1))/2 ? I've read through the GMAT Official Guide Math Review but don't recall learning that. It seems if you know that formula the question is very easy, but if you don't the problem can be a big time drain. Do you recommend other guides I should be reading to study up on tips and formulas for problems like these?

If you don't recall the formula, there is one method:

1 + 2 + 3 + 4 + 5 ........ + 26 + 27 + ............ + 48 + 50 + 51 + 52

Sum of 1st & last digit = 1 + 52 = 53

Sum of 2nd & 2nd last digit = 2 + 51 = 53

Sum of 3rd & 3rd last digit = 3 + 50 = 53
.
.
.
.
This is repeated 26 times

So, 26 * 53 = 1378

Yes! Definitely better/easier/faster to apply logic than to memorize another formula. This way, you can use the logic even if you don't start at one. Or if you are counting by 2's or 3's. For example, what if Nancy saved $51 the first week of the year, and increased her savings by$1 each week after that.

Start at $51. Know that there are 52 weeks in a year, so the last week she will save 102. So the sequence will be 51, 52, 53... 100, 101, 102. See how adding the first and last is the same as adding the second and the second-to-last, and the same as the third and third-to-last? All like pairs are$153.
If there are 52 items, then there are 26 pairs.
$153 x 26 =$3978

Counting by 3's:
First week is $51. Last week will be (52 - 1) x 3 = 153 more, or 51+153=204. So: 51, 54, 57... 198, 201, 204. First + last: 51 + 204 =$255.
Times 26 pairs: $255 x 26 =$6630

Counting by 3's, odd number of weeks:
Let's say Nancy only saved for 51 weeks, because she bought holiday presents for everyone in the last week.
First week is $51. Last week will be (51 - 1) x 3 = 150 more, or 51+150=201. So: 51, 54, 57... 195, 198, 201. First + last:$51 + $201 =$252.
Now, there are 25 pairs + 1. The one is in the very middle. So $252 x 25 =$6300. Still have to add that last number that wasn't part of a pair. What was it?
Nancy saved for 51 weeks, so that's 25 weeks on one side, 25 on the other, the middle is the 26th week.
26th week is (26 - 1) x $3 =$75 more than week 1, or 51 + 75 = 126.
$6300 +$126 = $6426 Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 6062 Location: Pune, India Followers: 1596 Kudos [?]: 8930 [0], given: 195 Re: In the first week of the Year, Nancy saved$1. [#permalink]  04 Jun 2014, 19:04
Expert's post
Puzzler wrote:

Yes! Definitely better/easier/faster to apply logic than to memorize another formula. This way, you can use the logic even if you don't start at one. Or if you are counting by 2's or 3's. For example, what if Nancy saved $51 the first week of the year, and increased her savings by$1 each week after that.

It's certainly better to apply logic than just learn up formulas for specific situation because you may find you have no formula for a given situation in the question. Also, you need to know exactly when the formula is applicable for example here you must know that the formula is applicable for first n positive integers only.

That said, n(n+1)/2 is a very basic and useful formula. You should know it for GMAT, not because you may not be able to do a question without it but because you may spend unnecessary amount of time on a question when other people will be able to run away on it with the formula.
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This is an evenly spaced set since the difference between terms is always 1.

Calculate the number of terms: 52 (last) - 1 (first) + 1 = 52
Calculate the mean: (52+1) / 2 = 26.5
Multiply: 26.5 * 52 = 1,378
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