Answer:
Why are you confused? EP said that the angle at the vertex that they(the equilateral triangle and the square share) is a right angle, or 90 degrees . Then the middle angle of the equilateral triangle is 60 degrees. Why? Because "equilateral" triangle has 3 "equal" sides and, therefore, 3 "equal" angles!!. Since the sum of the 3 angles of ANY triangle is 180 degrees, then 180/3 =60 degrees for each angle. Since the angle at the vertex is "splitting" a right angle, or 90 degrees which belongs to the square, then that leaves 2 smaller angles on each side of the 60-degree angle, or 90 - 60 =30/2 =15 degrees. That is the top angle of the triangle you are trying to solve.
Since that small triangle is a RIGHT triangle, then: 180 - 90 - 15 =75, which is the angle with a question mark?
