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An idea to solve this DS question??

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Status: GMAT on july 13.
Joined: 10 Feb 2017
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GMAT 1: 650 Q48 V30
An idea to solve this DS question??  [#permalink]

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New post 01 Jul 2017, 23:45
For statement one, two numbers 3 and 31 are YES.
but, when I started checking for all the prime numbers,
3,5,7,11,13.....
I ended up thinking it satisfies only for 3 after checking till 17.

there is clearly no pattern :? :(
it was a silly mistake, but because of time constraints we might not check till 31.

so, is there any other trick to solve for statement ??

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An idea to solve this DS question??  [#permalink]

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New post 02 Jul 2017, 01:03
1
S1 -> 2(x+1) is a cube of a positive integer so the quickest thing to do it start checking for positive even cubes, as 2(x+1) cannot be odd. So x = 3, 31,... Insufficient.

2^3 = 8 => x = 3
4^3 = 64 => x = 31

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Re: An idea to solve this DS question??  [#permalink]

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New post 02 Jul 2017, 07:32
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Re: An idea to solve this DS question??  [#permalink]

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New post 02 Jul 2017, 10:41
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Hi Viserion99,

To start, you should really post these 'subject-specific' questions in the proper forums. The DS forum can be found here:

https://gmatclub.com/forum/gmat-data-su ... cy-ds-141/

To answer your question though, you have to consider which 'part' of the prompt to focus on first. You chose to focus on the "prime" and try to find the 'cube'... but what if you had focused on the 'cube' and "worked backwards" to see if there was a prime involved.

For example, we know that (2X+2) is the cube of a POSITIVE INTEGER... so that cube COULD be...
1^3 = 1
2^3 = 8
3^3 = 27
4^3 = 64

With just these four examples, you can find three different primes (X=3, X=17 and X=31).

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Re: An idea to solve this DS question??   [#permalink] 02 Jul 2017, 10:41
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