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Thread: Number Properties: prime number addition

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    An Urch Guru Pundit Swami Sage
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    Number Properties: prime number addition

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    how to solve it a faster way, I was unable to do it in less than 2 mins, coz i had to list down all the prime number

    OA
    SPOILER: is D
    Attached Thumbnails Attached Thumbnails Number Properties: prime number addition-q23.jpg  

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    An Urch Guru Pundit Swami Sage manish8109's Avatar
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    All prime numbers are odd except 2 so,

    odd+odd=even
    even+odd=odd
    so, check for 19 it is 2+17=19(both 2 & 17 are prime)
    check for 45 it is 2+43=45 (both 2& 43 are prime)
    check for 68 as it is even so both prime number if applicable should be odd 61+7=68 (both 61 & 7 are prime)
    check for 79 as it is an odd number it means one of the prime number should be even i.e. 2+77=79 but 77 is not prime so Ans is 79-----(D)
    Check for 88 (even) means both prime number should be odd i.e. 83+5=88(both 5 & 83 are prime)

    Ans is (D)

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    An Urch Guru Pundit Swami Sage
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    Hey Manish!! Nice to see you back on forum and that too replying to my posts So Manish I guess you would have completed you MBA??

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    An Urch Guru Pundit Swami Sage
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    Quote Originally Posted by Gmater-1 View Post
    how to solve it a faster way, I was unable to do it in less than 2 mins, coz i had to list down all the prime number

    OA
    SPOILER: is D
    Start with the odd numbers, because you know that an odd sum of two integers has to have one even integer and one odd integer.

    Since 2 is the only even prime, it HAS to be one of the numbers for the odd choices. Therefore, subtract 2 from each odd choice, and whichever result is not prime is your answer.

    A. 19 - 2 = 17 PRIME

    B. 43 - 2 = 43 PRIME

    D. 79 - 2 = 77 NOT PRIME

    D is the answer.

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