Practice GMAT Data Sufficiency Question

In triangle ABC, what is the measurement of angle C?

A)	Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B)	Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C)	BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D)	EACH statement ALONE is sufficient.
E)	Statement (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data are needed.

Correct Answer: B

Since the sum of the measure of the interior angles of a triangle equals 180 degrees, you can write the following equation:
The measure of angles A + B + C = 180
Evaluate Statement (1) alone.
1. Translate Statement (1) into algebra:
  A + C = 120
2. Use the foundational triangle equation (i.e., all angles add up to 180):
  A + B + C = 180
  (A + C) + B = 180
  Substitute A + C = 120 into the equation.
  120 + B = 180
  B = 60
3. It is impossible to determine the value of angle C. Angle A could be 60 degrees and angle C could be 60 degrees. However, angle A could be 20 degrees and angle C could be 100 degrees.
4. Statement (1) is NOT SUFFICIENT.
Evaluate Statement (2) alone.
1. Translate Statement (2) into algebra:
  A + B = 80
2. Use the foundational triangle equation (i.e., all angles add up to 180):
  A + B + C = 180
  (A + B) + C = 180
  Substitute A + B = 80 into the equation.
  80 + C = 180
  C = 100
3. Statement (2) is SUFFICIENT.
Since Statement (1) alone is NOT SUFFICIENT and Statement (2) alone is SUFFICIENT, answer B is correct.