You are given a right triangle. The square in the corner of the triangle tells you that that angle = 90°. Also the sum of all three angles in a triangle = 180°.
Sum of the angles in the triangle = 40° + 90° + y = 180°
130° + y = 180°
y = 180° - 130°
y = 50°
Second way:
The acute angles of a right triangle = 90°. Therefore 40° + y = 90°
