DS - isoscel triangle angle

[Xbond]

Hi there,

Could you give me your ELEGANT, QUICK, and EFFICIENT method to resolve (with these concepts and with finding the solution).


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

[g1988enator]

sum of angle of triangle is 180.
if t is 100 so other 2 have 2 be 40 degree since it's a isosceles triangle so 1 statement is sufficent enough .
but 2 condition is not enough as angles can be 40 40 100 or 40 70 70

1 statemnt is enough

[Xbond]

many thks

OA is A