Explanation
The three angles in any triangle will always sum up to 180 degrees. A triangle with two equal sides will also have two equal angles. Hence,
The sum of all angles = angle 1 + angle 2 + angle 3
180 = (2 equal angles) + 120
2 equal angles = 180 – 120 = 60 degrees
Since angle 1 and angle 2 are two equal angles, 60 divided by 2 = 30 degrees