Question

An exterior angle of a triangle is 80 and two interior opposite angles are equal. What is measure of each angle.

Answer 1

We know that
Sum of the two opposite interior angle = exterior angle
So
Suppose that interoir angle = x
x+x= 80
2x = 80
x= 40
Interior angle = 40 °

Answer 2

we know that 
80+x=180
so,x=100 deg
now as we know 1 angle of the triangle and as two interior opp angles are equal,
100+x+x=180(angle sum property)
100+2x=180
2x=80
therefore x=40deg