Geometry

[gmat_junior]

In isosceles triangle RST what is the measure of angle R?
1) The measure of angle T is 100
2) The measure of angle S is 40

Answer is A but in my opinion answer should be D not A. Can anybody explain me plzz?

[mankuthimma]

Answer is not D because, ^R and ^T can be 70 each while ^S is 40

[pepeprepa]

1) The measure of angle T is 100
The triangle is isocele so two angles have the same value. T is 100, and given you have 180° in a triangle, the other two angles have the same values:40 each
Sufficient.

2) The measure of angle S is 40
Angle S is 40. We could have S=40 T=40 and R=100 or S=40 T=100 or R=40 or S=40 T=70 R=70
Insufficient.