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Source: — Problem Solving |

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by Brent@GMATPrepNow » Mon Oct 21, 2013 7:02 am
AIM TO CRACK GMAT wrote:What comes next in the following series? 1/21, 1/14, 1/18, 1/16, 1/15, 1/18, 1/12, ......

[spoiler]OA= 1/20[/spoiler]
This would never be an actual GMAT question because it's possible to find more than 1 pattern in a set of numbers.
For example, what comes next in this sequence: 1, 2, 4, ... ?
Well, perhaps we're doubling each number, in which case the next value is 8
Or, perhaps we're adding 1, then 2, then 3 then 4 (etc), in which case the next value is 7

Likewise, for 1/21, 1/14, 1/18, 1/16, 1/15, 1/18, 1/12, ......, I could say that the correct answer is 3/17, because 1/21, 1/14, 1/18, 1/16, 1/15, 1/18, 1/12 are my 7 favorite fractions and 3/17 is my 8th favorite :-)

Yes, I can see that having 7 values reduces the number of possible scenarios, but the rationale for why the GMAT would not test this still stands.

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Brent
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by ROHIT11111 » Tue Nov 03, 2015 3:24 am
the answer is wrong as the answer is 1/20 ... can u let us know the soln.
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by theCEO » Tue Nov 03, 2015 7:19 pm
AIM TO CRACK GMAT wrote:What comes next in the following series? 1/21, 1/14, 1/18, 1/16, 1/15, 1/18, 1/12, ......

[/spoiler]
ROHIT11111 wrote:the answer is wrong as the answer is 1/20 ... can u let us know the soln.
Notice that all the the numbers have 1 in the numerator, therefore answer will have 1 in the numerator.

Next find the factors of each number.

21 = 3 x 7
14 = 2 x 7
18 = 3 x 6
16 = 2 x 8
15 = 3 x 5
18 = 2 x 9
12 = 3 x 4

Notice that the first number in the red font is 3 and the other number starts at 7 and decreases by one.

Notice that the first number in the black font is 2 and the other number starts at 7 and increases by one.

Since the series go red,black,red, black.... the next number will be part of the black series which is 2 x (9+1) = 20

Since 1 is the numerator, the answer is 1/20
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by [email protected] » Wed Nov 04, 2015 10:14 am
HI AIM TO CRACK GMAT,

As Brent has pointed out, this is NOT representative of what you'll see on the GMAT. It looks more like a 'brainteaser' from some type of puzzle/math book. If you're actually studying for the GMAT, then you should focus on realistic (and reputable) GMAT practice resources.

GMAT assassins aren't born, they're made,
Rich
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