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For any positive integer n, the sum of the first n positive integers

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For any positive integer n, the sum of the first n positive integers  [#permalink]

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New post 09 Mar 2019, 14:20
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For any positive integer n, the sum of the first n positive integers equals n(n+1)/2. What is the sum of all the integers between 250 and 350, inclusive?

(A) 100
(B) 6,600
(C) 20,200
(D) 30,300
(E) 55,000
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For any positive integer n, the sum of the first n positive integers  [#permalink]

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New post Updated on: 09 Mar 2019, 15:51
Isn't the right answer 30.050?

Sum of the first 350 positive integers

\(\frac{350(350+1)}{2}=61.425\)

Sum of the first 250 positive integers

\(\frac{250(250+1)}{2}=31.375\)

Edit:

Doing the difference between the two:

\(62.750-31.375=30.050\)

Adding back the 250 who was inside the 31.375:

\(30.050+250=30.300\)

Originally posted by Mrpd on 09 Mar 2019, 14:36.
Last edited by Mrpd on 09 Mar 2019, 15:51, edited 1 time in total.
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Re: For any positive integer n, the sum of the first n positive integers  [#permalink]

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New post 09 Mar 2019, 14:52
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Mrpd wrote:
Isn't the right answer 30.050?

Sum of the first 350 positive integers

\(\frac{350(350+1)}{2}=61.425\)

Sum of the first 250 positive integers

\(\frac{250(250+1)}{2}=31.375\)

Sum of all the integers between 250 and 350

\(62.750-31.375=30.050\)


How are you getting such small numbers?

Check this post by Bunuel: https://gmatclub.com/forum/for-any-posi ... l#p1046839
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Re: For any positive integer n, the sum of the first n positive integers  [#permalink]

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New post 09 Mar 2019, 15:45
energetics wrote:
Mrpd wrote:
Isn't the right answer 30.050?

Sum of the first 350 positive integers

\(\frac{350(350+1)}{2}=61.425\)

Sum of the first 250 positive integers

\(\frac{250(250+1)}{2}=31.375\)

Sum of all the integers between 250 and 350

\(62.750-31.375=30.050\)


How are you getting such small numbers?

Check this post by Bunuel: https://gmatclub.com/forum/for-any-posi ... l#p1046839


Sorry, you're right.

I've excluded the 250 of the sum when I did \(62.750-31.375=30.050\).

The correct answer is 30.300 indeed.
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Re: For any positive integer n, the sum of the first n positive integers   [#permalink] 09 Mar 2019, 15:45
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