very interesting and alike the way I approached this question which has answer choices (not posted) and hence is limited to the given no-s
Nevertheless, I found the largest integer as per rule:
the total deviations below the mean should be equal to the total deviations above the mean
I set mean as the value greater than the fourth no (the only possible, because the fifth cannot be greater

) and obtained
a, a+C, a+C+1, a+C+2, E
a is the first term, C is the constant (likewise number with zeros in Nil's solution, 100000) and I accrued everything by 1 (the different positive integers), E is the largest integer
As the result I have obtained the function >>>
mean=a+a+C
Deviations below the mean:
a+a+C-a=a+C
a+a+C-(a+C)=a
a+a+C-(a+C+1)=a-1
a+a+C-(a+C+2)=a-2
summing up (finding total deviations)=4a+C-3
E=4a+C-3 +(a+a+C)=6a+2C-3
To simplify, if we are not using C as the constant, then we get the largest integer E=6a-1
and you are right, this set without the given answer choices is meaningless for GMAT as it continues to accrue and accrue the largest integers
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