Final answer:
In an isosceles triangle, if one of the base angles is 80°, the value of the vertical angle is 20°.
Step-by-step explanation:
An isosceles triangle has two equal base angles. If one of the base angles is 80°, then the other base angle is also 80°. The sum of the angles in a triangle is always 180°. Therefore, the value of the vertical angle is 180° - 80° - 80° = 20°.