GmatGreen wrote:How many integers n are there such that r < n < s ?

1) s - r = 5
2) r and s are not integers

Target question: How many integers n are there such that r < n < s?

Statement 1: s - r = 5
There are two cases we need to consider.
Case a: s and r are integers.
For example, s=6 and r=1, in which case there are 4 integers between r and s (2, 3, 4 and 5)
Case b: s and r are not integers.
For example, s=6.1 and r=1.1, in which case there are 5 integers between r and s (2, 3, 4, 5, and 6)

Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: r and s are not integers
This is definitely not enough information here to answer the target question.
Consider these 2 cases.
Case a: r=1.1 and s=2.1, in which case there is 1 integer between r and s (2)
Case b: r=1.1 and s=3.1, in which case there are 2 integers between r and s (2 and 3)

Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined:
From statement 1, we know that there are either 4 or 5 integers between r and s, depending on whether or not r and s are integers.
Statement 2 rules out the possibility that r and s are integers.
If r and s are non integers, then there must be 5 integers between r and s

Since we can answer the target question with certainty, the combined statements are SUFFICIENT

Answer = C

Cheers,
Brent

Hi GmatGreen,

As everyone else has already mentioned, thinking about how the answer changes when dealing with integers vs. non-integers is necessary on this question.

Going forward with DS questions, here are some things to note:

1) You won't have to consider decimals and fractions all that often (that level of depth is only required on a handful of DS questions)
2) Look for any "clues" (such as how Fact 2 was written) and "limitations" (the phrase X > 0 means that X cannot be 0 or negative, so what's left? - positive integers and positive fractions) appear in the question to guide you.

GMAT assassins aren't born, they're made,
Rich