From the given figure ,
The exterior angle is 100°
One of the interior angle is 40° and the other is given to be x
Now let the angle adjacent to 100° be y,
If we observe carefully, it is a straight line and hence has the sum of angles to be 180°
So the angle y is calculated as :
100 + y = 180°
=> y = 180-100
=> y = 80°
Now we know two angle in the triangle
They are 40 and 80 degrees each.
As we know , sum of the angles in a triangle is equal to 180°
Let’s equate sum to 180°
=> x + 40 + 80 = 180°
=> x + 120 = 180°
=> x = 180 -120
=> x = 60° is the required answer.