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problem solving

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problem solving

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A set of numbers has the property that for any number t in the set, t + 2 is in the set. If -1 is in the set, which of the following must also be in the set?
I.-3

II.1

III.5
A I only
B II only
C I and II only
D II and III only
E I, II, and III

OA: D

I doubt why -3 is not in the list?

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GMAT/MBA Expert

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fiveflowers wrote:
A set of numbers has the property that for any number t in the set, t + 2 is in the set. If -1 is in the set, which of the following must also be in the set?
I.-3

II.1

III.5
A I only
B II only
C I and II only
D II and III only
E I, II, and III

OA: D
If t is in the set, then t+2 is in the set.
Since -1 is in the set, -1+2 = 1 is in the set.
Since 1 is in the set, 1+2 = 3 is in the set.
Since 3 is in the set, 3+2 = 5 is in the set..

Since the set must contain both 1 and 5 (statements II and III), eliminate A, B, and C.

If t is in the set, then t+2 is in the set.
The portion in red tells us ONLY that t+2 must be in the set.
It tells us NOTHING about values LESS than t: they could be in the set, or they could not be in the set.
Thus, given that -1 is in the set, we cannot determine whether any values LESS than -1 are in the set.
Since -3 (statement I) does not have to be in the set, eliminate E.

The correct answer is D.

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