In isosceles triangle RST what is the measure of angle R?

gmatgg,

This is a fairly one simple one that you should spend no more than 30 seconds on.

The keyword is "isosceles"---which means 2 sides are the same and also...2 angles are the same

With statement (1), you are told measure of angle T is 100 degrees. You already know an entire triangle = 180 degrees.

So either one of the other angles also equals 100 degrees, or the other two angles not mentioned are equivalent.

Well, it's impossible for one of the other angles to also be 100 degrees, because that would exceed that total of 180 degrees possible in a triangle.

So the only choice that makes sense is to have the other two angles equal and everything adds to 180.

In other words, the other 2 angles combined must equal the difference between 180 and 100.
Mathematically, 180 - 100 = 80 and then divide that by 2 to get 40 degrees.

So (1) by itself is good enough.

Statement (2) is just telling us that one angle is 40 degrees. Well, what possibilities are there? You could have one 40 degrees and then the other two angles are equivalent and all 3 add up to 180. OR you can have another angle that is also 40 degrees and the third angle would add up to 180.
In other words, you'd have two possibilities:

1) 40 - 70 - 70 (adds up to 180)
or 2) 40 - 40 - 100 (also adds up to 180)

Since you have MULTIPLE possibilities with statement (2), then you do not have enough information to definitively say what the answer is. So Statement (2) is not good enough.

With (1) good and (2) no good, answer is (A).

Hope that helps.