Find the degree measure of the angle B in the isosceles triangle ABC with the base AC if the outer angle at the apex A is 130 degrees.

If the adjacent angle of the triangle at the vertex of angle A is 130 °, then the angle A is 50 °, since the adjacent angles add up to 180 °, according to the property of adjacent angles. Since the triangle ABC is isosceles, the angles at its base are equal, A = C = 50 °. Since the sum of all the angles in any triangle is 180 °, we find the desired angle B = 180 ° – A – C = 180 ° – 50 ° – 50 ° = 80 °.