You have to use the property of alternate angles of parallel lines Alternate angles are equal: Sometimes called Z angles We can often spot interior alternate angles by drawing a Z shape From the diagram above, angle ABC= angle CDE=x (Alternate angles) Angle BAC=y=DEC (Alternate angles) Angle ABC= angle CDE=x (Alternate angles) Angle ABC=81∘,∴x=81∘ Since we know the angles DCE and CDE, we can calculate the angle DEC using the concept that angles in a triangle add up to 180∘ x=81∘ and DCE=20∘
Sum of angles in triangle add up to 180∘ 81+20+ angle DEC=180 101+DEC=180 DEC=180−101 DEC=79 Since BAC=y=DEC,y=79∘