Data Sufficiency

[email protected] wrote:In an isosceles triangle RST, what is the measure of angle R?

1. The measure of angle T is 100 degrees.
2. The measure of angle C is 40 degrees.

"C" must be S.

As RST is an isosceles triangle, two of the three angles will have equal measures. As sum of the three angles will be 180 degrees, the measure of these equal angles must be less than 90 degrees.

Statement 1: Measure of angle T is 100 degrees > 90 degrees.
Hence, T is not one of the equal angles.

Hence, R = S = (180 - 100)/2 degrees = 40 degrees

Sufficient

Statement 2: Measure of angle S is 40 degrees
Hence, S may or may not be one of the equal angles and we cannot draw any further conclusions.

Not Sufficient

The correct answer is A.