# Thread: Number Properties: prime number addition

1. Good post? |

## Number Properties: prime number addition

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

2. Good post? |
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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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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Originally Posted by Gmater-1
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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