Data Sufficiency

Gmat_mission wrote:If x and y are two points on the number line what is the value of x + y?

(1) 6 is halfway between x and y.
(2) y = 2x

Target question: What is the value of x + y?

Statement 1: 6 is halfway between x and y.
KEY CONCEPT: The average (arithmetic mean) of 2 numbers is HALFWAY between those 2 numbers.
For example, the average of 1 and 9 is 5. Notice that 5 is HALFWAY between 1 and 9.

So, statement is telling us that 6 is the average of x and y
In other words, (x + y)/2 = 6
This means x + y = 12
The answer to the target question is x + y = 12
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: y = 2x
There are infinitely many values of x and y that satisfy statement 2. Here are two:
Case a: x = 1 and y = 2. In this case, the answer to the target question is x + y = 1 + 2 = 3
Case b: x = 3 and y = 6. In this case, the answer to the target question is x + y = 3 + 6 = 9
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer: A

Cheers,
Brent