GMAT Official Guide 2019 What is the circumference

[BTGmoderatorDC]

What is the circumference of circle C ?

(1) The radius of circle C is 2Ï€.
(2) The center of circle C is located at point (7,8) in the xy-plane.

[Brent@GMATPrepNow]

What is the circumference of circle C ?

(1) The radius of circle C is 2Ï€.
(2) The center of circle C is located at point (7,8) in the xy-plane.

Target question: What is the circumference of circle C ?

Statement 1: The radius of circle C is 2Ï€.
Useful formula: circumference = (2Ï€)(radius)
Now that we know the radius is 2π, the circumference = 2π(2π) = 4π²
In other words, the answer to the target question is the circumference of circle C is 4π²
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: The center of circle C is located at point (7,8) in the xy-plane.

Notice that there are infinitely many different circles that can have their center at (7,8)
This means there are infinitely many different answers to the target question
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer: A

Cheers,
Brent