Quick tips for adding numbers x to y : GMAT Quantitative Section
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# Quick tips for adding numbers x to y

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Manager
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10 Jun 2009, 19:46
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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, 18:59, edited 7 times in total.
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10 Jun 2009, 21:58
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.
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12 Jun 2009, 13:34
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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...
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30 Jun 2009, 19:11

Thanks nookway, your help is appreciated!
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03 Jul 2009, 06:51
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Expert's post
I3igDmsu wrote:

Thanks nookway, your help is appreciated!

Don't think you need our help - this can be easily done in Excel
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05 Jul 2009, 19:00
Done. If anyone needs practice, the answers in the spoiler are correct.
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02 Aug 2009, 20:30
Great stuff! Thanks!
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13 Oct 2011, 13:32
I did not get the logic of these.
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03 Feb 2015, 11:25
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16 Jul 2016, 23:59
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: Quick tips for adding numbers x to y   [#permalink] 16 Jul 2016, 23:59
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