If one of the angles is 100 then if the triangle is isosceles then the two remaining angles have to be either 100 and - 20 (which is impossible because at this level of math there is no such thing as an angle with a measure of - 20).
The minus 20 comes from every triangle having 180 degrees. 100 + 100 adds to 200, which means the third angle has to be the impossible measure of - 20.
The other possibility is that the given angle of 100 is the top angle and that leaves 80 degrees for the two isosceles angles.
