Data Sufficiency (DS) | Re: Is 15 the average (arithmetic mean) of x, y, and 15?

Average of x, y and 15 is = (x+y+15)/3.If this is equal to 15, then we will have:
x+y+15 = 45 OR x+y = 30
So if we can determine whether x+y is 30 or not (with surety) then we will be able to answer.

Statement 1
This gives x+y = 30.Sufficient . This gives us YES as an answer to the question asked.

Statement 2 x-y = 4. This does not help us in determining whether sum of x & y is 30 or not. So not sufficient
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