NOTE: I added the necessary information to the question below.

nakul17 wrote:If s@t = ts/(t+s), where both s and t are prime numbers, does s@t = 1?

(1) s + t ≠ 4
(2) s/t ≠ 1

This is a great candidate for rephrasing the target question. In fact, if we spend a bit of time rephrasing the target question, we can solve the question quickly. (Aside: We have a free video with tips on rephrasing the target question: https://www.gmatprepnow.com/module/gmat- ... cy?id=1100)

Target question: Does s@t = 1?
In other words, does ts/(t+s) = 1?
We can also word this as: does (t+s)/ts = 1?
Or we can write: does t/ts + s/ts = 1?
Simplify to get: does 1/s + 1/t = 1?

IMPORTANT: since s and t are prime numbers, we know that s > 2, and t > 2
When s = 2 and t = 2, it's true that 1/s + 1/t = 1
If s > 2 and t > 2, then 1/s < 0.5 and 1/t < 0.5. Since it's impossible to two values less than 0.5 to add to 1, we can see that 1/s + 1/t ≠ 1 if s > 2 and t > 2

In other words, THE ONLY WAY that 1/s + 1/t can equal 1 is if s = 2 and t = 2.
So, we can rephrase our target question as . . .
REPHRASED target question: Does s = t = 2?

Statement 1: s + t ≠ 4
If s + t ≠ 4, then it is IMPOSSIBLE for s = t = 2
Since we can answer the REPHRASED target question with certainty, statement 1 is SUFFICIENT

Statement 2: s/t ≠ 1
If s/t ≠ 1, then it is IMPOSSIBLE for s = t = 2
Since we can answer the REPHRASED target question with certainty, statement 2 is SUFFICIENT

Answer = D

Cheers,