Data Sufficiency

Not sure if I'm missing something here but...

If triangle RST is an isosceles, what is angle R?
1) angle T = 100
2) angle S = 40

I thought for an isosceles, if you knew the middle angle; [180-middle angle] /2 would be each remaining angle no?

Apparently the answer is NOT choice D....

Sorry I know it sounds like a silly question and it was from the gmatprep as well..

Statement 1 : If one angle is 100; that means in an isosceles triangle the other angles are 40 each. ;Sufficient

Statement 2 : One angle is 40. Now this is the trick. Since we do not know which sides are equal, we cannot say which angles are equal. If one angle is 40 it might mean:
a. 40-40-100;
b. 40-70-70;
;Hence not sufficient.

--> Answer is A

If triangle RST is an isosceles, what is angle R?

1) angle T = 100

Angle R + Angle S + Angle T = 180
Case 1: Angle R = Angle T, then
Angle R + Angle T = 200 > 180 (Sum of Angles R,S and T).
Not possible
Case 2: Angle R = Angle S, then
Angle S + Angle S + Angle T = 180
Angle R = Angle S = 40, Angle T = 100
Angle R = Angle S = 40.
Sufficient

2) angle S = 40

Angle R + Angle S + Angle T = 180
Case 1: Angle R = Angle T, then
Angle R + Angle S + Angle R = 180
Angle R = Angle T = 70, Angle S = 40
Case 2: Angle R = Angle S, then
Angle S + Angle S + Angle T = 180
Angle R = Angle S = 40, Angle T = 100
Two different cases, Oops Insufficient!

IMO A