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# Quick tips for adding numbers x to y

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Manager
Joined: 25 May 2009
Posts: 141
Concentration: Finance
GMAT Date: 12-16-2011
Quick tips for adding numbers x to y [#permalink]

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10 Jun 2009, 20:46
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Here are some examples on adding numbers from x to y:

1) Add the numbers from 40 to 70, inclusive.
2) Add the even numbers from 40 to 70, inclusive.
3) Add the odd numbers from 40 to 70, inclusive.

4) Add the numbers from 40 to 70.
5) Add the even numbers from 40 to 70.
6) Add the odd numbers from 40 to 70.

[Reveal] Spoiler:
1) 1,705
2) 880
3) 825
4) 1,595
5) 770
6) 825

Last edited by I3igDmsu on 05 Jul 2009, 19:59, edited 7 times in total.
Manager
Joined: 28 Jan 2004
Posts: 201
Location: India
Re: Quick tips for adding numbers x to y [#permalink]

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10 Jun 2009, 22:58
1) Add the numbers from 40 to 70, inclusive.
Formula for sum of first N natural numbers is = N(N+1)/2
Sum from 1 to 70 Sa= 70*71/2
Sum from 1 to 40 Sb= 40*41/2
So sum from 40 to 70 = Sa-Sb

2) Add the even numbers from 40 to 70, inclusive.
3) Add the odd numbers from 40 to 70, inclusive.

========================

As a general solution to all these kind of problems learn the AP series.
google on Arithemic Progression series. It is kind of difficult to write the formula here but all these calculations are tooooooo simple using this series.
Current Student
Joined: 03 Aug 2006
Posts: 112
Location: Next to Google
Schools: Haas School of Business
Re: Quick tips for adding numbers x to y [#permalink]

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12 Jun 2009, 14:34
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If you don't mind remembering a formula or two then yes you can.
The examples that you have given can be grouped under Arithmetic Progressions...or a finite sequence of evenly spaced numbers.

There are two formulas:

$$1. S = \frac{n}{2} [2a + (n-1)d]$$

where
$$S=\text{sum of the all the numbers in the sequence}$$
$$n=\text{total number of numbers in the sequence}$$
$$a=\text{the first number of the sequence}$$
$$d=\text{the different between any two consecutive numbers in the sequence}$$

$$2. S = \frac{n}{2} [\text{First Term}+\text{Last Term}]$$

where
$$S=\text{sum of the all the numbers in the sequence}$$
$$n=\text{total number of numbers in the sequence}$$

You can use either equation based on what is provided in the question.

Lets take your examples and solve them. For all of these we know the first and the last number so we should be fine with equation 2.

1) Add the numbers from 40 to 70, inclusive.
Solution:
Here the sequence is 40,41, 42, ...., 69, 70.
To calculate n
$$n=\text{Last number}-\text{First number} + 1$$

$$n=70-40+1 = 31$$

Using equation 1.

$$S = \frac{n}{2} [\text{First Term}+\text{Last Term}]$$

$$S = \frac{n}{2} [\text{First Term}+\text{Last Term}]$$

$$S = \frac{31}{2} [40+70]$$

$$S = \frac{31}{2} [110]$$

$$S = 31\times55$$

$$S = 1705$$

2) Add the even numbers from 40 to 70, inclusive.
Here the sequence is 40,42, 44, ...., 68, 70.
Solve using 40 as the first term and 70 as the last term
To calculate n for even (or odd) number

$$n=\frac{\text{Last number}-\text{First number}}{2} + 1$$

3) Add the odd numbers from 40 to 70, inclusive.
Here is the sequence is 41,43,....67,69
Solve using 41 as the first term and 69 as the last term

and so on...
Manager
Joined: 25 May 2009
Posts: 141
Concentration: Finance
GMAT Date: 12-16-2011
Re: Quick tips for adding numbers x to y [#permalink]

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30 Jun 2009, 20:11
Could someone check my answers above please?

Thanks nookway, your help is appreciated!
Founder
Joined: 04 Dec 2002
Posts: 16588
Location: United States (WA)
GMAT 1: 750 Q49 V42
Re: Quick tips for adding numbers x to y [#permalink]

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03 Jul 2009, 07:51
1
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Expert's post
I3igDmsu wrote:
Could someone check my answers above please?

Thanks nookway, your help is appreciated!

Don't think you need our help - this can be easily done in Excel
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Manager
Joined: 25 May 2009
Posts: 141
Concentration: Finance
GMAT Date: 12-16-2011
Re: Quick tips for adding numbers x to y [#permalink]

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05 Jul 2009, 20:00
Done. If anyone needs practice, the answers in the spoiler are correct.
Manager
Joined: 13 Jan 2009
Posts: 168
Schools: Harvard Business School, Stanford
Re: Quick tips for adding numbers x to y [#permalink]

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02 Aug 2009, 21:30
Great stuff! Thanks!
Manager
Joined: 06 Feb 2011
Posts: 69
WE: Information Technology (Computer Software)
Re: Quick tips for adding numbers x to y [#permalink]

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13 Oct 2011, 14:32
I did not get the logic of these.
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Re: Quick tips for adding numbers x to y [#permalink]

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13 Sep 2017, 22:23
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Re: Quick tips for adding numbers x to y   [#permalink] 13 Sep 2017, 22:23
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# Quick tips for adding numbers x to y

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