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I have realised that understanding the principles behind the relations between numbers saves your time as compared to direct calculation.
Just a different way of looking at this question
Starting from 1 very third number is a multiple of 3. This means when you consider any 3 consecutive numbers, one (and only one) of them is a multiple of 3. (This is true for - to + infinity)
As the first equation says n & (n+1) are not multiples of 3, (n-1) & (n+1) have ot be multiples of 3. - SUFFICIENT
3n+5 >= k+8
As k is a multiple of 3, k+3 is also a multiple of 3 and as we don't have to find out the vaue of n or k, we can simply replace k+3 by k.
Hence:
3n+5 >= k+5
i.e. 3n >= k
This does not say anything at all!!! - NOT SUFFICIENT
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