Theorem:
The measure of an exterior angle of a triangle is equal to the sum of the measures of the remote interior angles.
<CAD is an exterior angle of triangle ABC.
Interior <CAB is adjacent to exterior <CDA.
Remote means far away.
The remote interior angles of <CAD are angles B and C.
m<CAD = m<B + m<C
m<CAD = 20 + 80
m<CAD = 100 deg
