Find the angles of a triangle, knowing that the outer angles at its two vertices are 120 and 150 degrees.

Taking into account all the above written reasoning and the fact that two of the three outer angles of the triangle are equal to 120 and 150 degrees, it means:

one angle of the triangle is 180 – 120 = 60 degrees;
the second angle of the triangle is 180 – 150 = 30 degrees;
since the sum of all angles in any triangle is 180 degrees, then the third angle of the triangle is 180 – 60 – 30 = 180 – 90 = 90 degrees, that is, this angle is right, which means this triangle is called rectangular.
Answer: the angles of the desired triangle are 60, 30 and 90 degrees.