A base angle of an isosceles triangle contains 75º. How large is the vertex angle? (A) 75°
(B) 30°
(C) 1050
(D) 210°
(E) 40°

Answer is B, there are two equal base angles in an isosceles triangle, so given that ( 75+75 =150), the total of the interior angles in any triangle is 180, so 180-150 =30 ( which is the vertex angle) , the upper angle of the triangle.