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bigfernhead · **Posted:** Fri Nov 14, 2008 3:02 pm


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What is the value of k if the sum of odd, consecutive, positive integers from 1 to k equals 441?

(A) 47
(B) 41
(C) 37
(D) 33
(E) 29

[Reveal] Spoiler: OA

B

Source: GMAT Club Tests - hardest GMAT questions

I'm using the formulas:

1. Sum of S = n/2(2a+(n-1)d) and
2. Sum of S = n/2(a+l)

where a = first number
l= last number
d= difference between the numbers

And I'm not getting the right answer.

nravi4 · **Posted:** Fri Aug 06, 2010 7:50 am

The sum of any evenly distributed numbers = (Total numbers in the distribution) * ((first number + lastnumber)/2)

Example1: The sum of first 5 integers (1, 2, 3, 4, 5) is:
(5)*((1+5)/2) = 5*3 = 15

Example2: The sum of first 5 odd integers (1 3 5 7 9) is:
5*((1+9)/2) = 5 * 5 = 25

Example3: The sum of first 6 odd integers ( 1 3 5 7 9 11) is: 6 * 6 = 36

========
What's given in the problem is the sum of 1 to k odd consecutive integers is: 441 (i.e: 7*7*3*3, i.e, 21*21)
===> 21 * (( 1+k)/2) = 21 * 21
===> (1+k)/2 = 21
===> k = 42 - 1 = 41

Cheers!
Ravi

debaranjansahoo · **Posted:** Thu Aug 05, 2010 8:08 am

Sn = (n/2){2a + (n-1)*d}
tn = a + (n - 1 ) * d

Sn = Sum of terms in the series
tn = nth term of the series
a = First term in series = 1
n = Number of terms in the series = is Unknown
d = Difference between consecutive terms in the series = 2

Sn = 441

so 441 = (n/2) * { 2 * 1 + (n-1) * 2}
From this n = 21 (discarding the negative value)

Question , it is asked k , which is the n th term

tn = t21 = 1 + (21 - 1) * 2 = 41
Answer is 41 (B)

Bidisha · **Posted:** Mon Nov 17, 2008 11:57 pm

sum of consecutive positive odd integers between 1 and k is (1/4)*(k+1)^2

from the sum of arithmetic progression S= n/2(2a+(n-1)*d)
put n=(k+1)/2 and d=2

therefore, (k+1)^2=1764
k+1=42
k=41

fluke · **Posted:** Mon Aug 08, 2011 6:31 am

bigfernhead wrote:

What is the value of k if the sum of odd, consecutive, positive integers from 1 to k equals 441?

(A) 47
(B) 41
(C) 37
(D) 33
(E) 29

Average of elements*Number of Elements=Sum of elements

\frac{k+1}{2}*(\frac{k-1}{2}+1)=441

k=41

Ans: "B"

hibloom · **Posted:** Fri Nov 14, 2008 4:57 pm

bigfernhead wrote:

The sum of the odd positive integers from 1 to k equals to 441. What is the value of k?

47
41
37
33
29

I'm using the formulas:

1. Sum of S = n/2(2a+(n-1)d) and
2. Sum of S = n/2(a+l)

where a = first number
l= last number
d= difference between the numbers

And I'm not getting the right answer.

you will get the answer using the first formula (arithmetic progression)

s=n/2(2a+(n-1)d)

where, n= number of terms
a=1 is the first term
d=2 is the common difference

solving , 441=n/2(2+ (n-1)2)= n^2 ==> n= 21 ==> number of terms = 21 ie k =41 as there are 21 positive consecutive intergars starting 1 hence 20 even on the way to 41. Hence answer is 41

icandy · **Posted:** Sat Nov 15, 2008 1:28 pm

bigfernhead wrote:

The sum of the odd positive integers from 1 to k equals to 441. What is the value of k?

47
41
37
33
29

I'm using the formulas:

1. Sum of S = n/2(2a+(n-1)d) and
2. Sum of S = n/2(a+l)

where a = first number
l= last number
d= difference between the numbers

And I'm not getting the right answer.

Here is what I did. The number 441 was really odd and some how it occurred to me that it is square of 21. So we need 21 21's in this sequence to get the sum to 441. We are starting with 1. The last number possibly is 41 because 1 and 41 average is 21. So are (3,39) (5,37) and so on.

Not a great way but helpful when we have strange numbers that have some significance and lets us connect dots.

alpha_plus_gamma · **Posted:** Tue Nov 18, 2008 1:31 am

Bidisha wrote:

therefore, (k+1)^2=1764
k+1=42

isn't calculating this time consuming?
(its fine for 42 as it is closer to 40, but numbers like 47 will need time)

bibha · **Posted:** Sat Jul 17, 2010 7:45 pm

I got N = 21
Could anyone tell me how we get k=41 and not 42???

mridulbr · **Posted:** Thu Aug 05, 2010 5:32 am

Math (PS)

What is the value of if the sum of odd, consecutive, positive integers from 1 to equals 441?

(A) 47
(B) 41
(C) 37
(D) 33
(E) 29

Explanation:

The sum is given as 441 which means by using the sum to n terms of an AP we can arrive at the following

n*(2a+(n-1)*d)/2 = 441

here, a=1 (given)
d=2 (consecutive odd integers)
k= a +(n-1)*d = 1+(n-1)*2 =2n-1

put these values in the above eqn and you'll get n=21. So the total number of terms in the AP = 21.
the value of k will be = 1+(21-1)*2 = 41 which is option A.

vittarr · **Posted:** Thu Aug 05, 2010 5:53 am

based on the following formulae:
sum of the first consecutive ODDS is N^2
sum of the first consecutive EVENS is N*(N+1)
=> 441=N^2 => N=21 => k=21*2-1=41

abhtiw · **Posted:** Thu Aug 05, 2010 6:11 am

somewhat different approach

sum of all consecutive numbers from 1....n is \frac{n*n+1}{2}

sum upto what such n is approx 441*2

find out that n so that \frac{n*n+1}{2} = 441 *2

so n*n+1 = 1764

looking at the answer choices 41 and 47 are the closest, and 41 more so

hmuhamma · **Posted:** Thu Aug 05, 2010 6:38 am

I used the answer choices and worked backwards knowing that since this is an evenly spaced set, the sum = midpoint or avg * number of items in the set.

In an evenly spaced set, the avg and midpoint = first + last/2 using the answer choices, where k equals 1, 1 + 41 =42/2 = 21. 21 should be midpoint. Second step was sum divided by midpoint or avg which we just found to be 21. So, 441/21 = 21 to find the number of items in the set.

Since sum = avg*number....441 = 21*21.

B is the correct answer. I was lucky in that the correct answer was earlier in the answer choices.

I dont understand the sum progression formula. Can some please help me on this one?

ankur17 · **Posted:** Thu Aug 05, 2010 7:58 am

An = A1 + (n-1)d

therefore
k = 1 + (n-1)2
n = (k+1)/2

Also
Sn = n/2 ( A1 + An)
441 = (K+1)/4 ( 1 + k )
k = 41

srivicool · **Posted:** Thu Aug 05, 2010 9:11 pm

even i knew only the sum of consecutive integers n(n+1)/2 .

thanks for the generic formula and explanation ...

zisis · **Posted:** Fri Aug 06, 2010 4:23 pm

nravi4 wrote:

The sum of any evenly distributed numbers = (Total numbers in the distribution) * ((first number + lastnumber)/2)

Example1: The sum of first 5 integers (1, 2, 3, 4, 5) is:
(5)*((1+5)/2) = 5*3 = 15

Example2: The sum of first 5 odd integers (1 3 5 7 9) is:
5*((1+9)/2) = 5 * 5 = 25

Example3: The sum of first 6 odd integers ( 1 3 5 7 9 11) is: 6 * 6 = 36

========
What's given in the problem is the sum of 1 to k odd consecutive integers is: 441 (i.e: 7*7*3*3, i.e, 21*21)
===> 21 * (( 1+k)/2) = 21 * 21
===> (1+k)/2 = 21
===> k = 42 - 1 = 41

Cheers!
Ravi

keep finding stuff i dont understand in explanations and for the first time i will try to point out something with the risk of looking stupi.....

whilst your examples are very good, why do you multiply by 21? I get that 441=21*21 but how do you know that
(Total numbers in the distribution) = 21?

zisis · **Posted:** Fri Aug 06, 2010 4:26 pm

abhtiw wrote:

somewhat different approach

sum of all consecutive numbers from 1....n is \frac{n*n+1}{2}

sum upto what such n is approx 441*2

find out that n so that \frac{n*n+1}{2} = 441 *2

so n*n+1 = 1764

looking at the answer choices 41 and 47 are the closest, and 41 more so

again, using the example formula, \frac{n*n+1}{2} = 441 and not =441*2
!!! y do you multiply by 2 the right hand side?

vittarr · **Posted:** Sat Aug 07, 2010 12:59 am

why you try to make it more complicated.

Variant #1
sum of the first consecutive odd integers is N^2. that's it.

Variant #2
If you want to make it complex: sum of consecutive integers is N*(a1+an)/2=N*(2*a1+D*(N-1))/2
where a1=1, D=2 =>
N*(2*1+2*(N-1))/2=N^2=441 and further as in the first variant.

mustdoit · **Posted:** Sat Aug 07, 2010 1:23 am

bigfernhead wrote:

What is the value of k if the sum of odd, consecutive, positive integers from 1 to k equals 441?

(A) 47
(B) 41
(C) 37
(D) 33
(E) 29

[Reveal] Spoiler: OA

B

Source: GMAT Club Tests - hardest GMAT questions

Sum of 1+3+5+7+9 = 25

Now every next 5 odd number will be greater by 50

for eg....11+13+15+17+19 = 75
21+23+25+27+29 = 125 and so on...

SO reaching 441 is like this...

25+75+125+175 = 400 ( from 1 to 40)

now adding 41 gives 441 so the K is 41....

No need to use formula......heh heh...

120secs · **Posted:** Mon Aug 08, 2011 6:12 am

Here is a non-algebraic solution

Put given solutions in ascending order:

29
33
37
41
47

TEST 37 (in this way we can eliminate 2 choices)

S=1+3+5+....+37
#of terms: (37-1)/2+1=19
mean: (37+1)/2=19
Sum of terms: 19*18= 342 < 441 (so we eliminate 29,33,37)

We can either try 41 and 47, or using common sense:

361+ 39 + 41 = 441

Answer is B. Any comments welcome

-120secs

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