adwaitkasbekar wrote:In isosceles triangle RST, what is measure of angle r?
a. Angle T is 100º
b. Angle S is 40º

Given: RST is an isosceles triangle. That means it has two equal angles and the third one is different. Now angle R may be one of the equal angles or the different one.

Statement 1: Angle T = 100º
As T > 90º, there is no way T is one of the equal angles. Thus R and S must be the equal angles. Which implies 2R = (180º - T) = (180º - 100º) = 80º => R = 40º

Sufficient.

Statement 2: Angle S = 40º
As S < 90º, S may be one of the equal angles or may be the different one.

Not sufficient.

The correct answer is A.