ANSWER
[tex]m \angle ACE = 80 \degree[/tex]
EXPLANATION
Recall that adjacent angles on a straight line will add up to 180°
From the diagram, m<ACD and m<ACE are adjacent angles on a straight line because they have a common vertex at C.
We sum the two angles and equate them to 180°
[tex] m \angle \: ACD + m \angle ACE = 180 \degree[/tex]
From the diagram
[tex]m\angle ACD= 100 \degree[/tex]
We substitute the known angle to obtain:
[tex]100 \degree + m \angle ACE = 180 \degree[/tex]
[tex]m \angle ACE = 180 \degree - 100 \degree[/tex]
[tex] \therefore \: m \angle ACE = 80 \degree[/tex]
