Angles a, b, and c are all 60°. A triangle’s angles add up to 180, and if the triangle is equilateral, each angle must be 60 (180/3). Angle d is 120° because b and d form a supplementary angle (180°), so 180-60 is 120. Angle e and f and 30°. Angles c and e form complementary angles (90°), so 90-60=30. For f, if the triangle adds up to 180, 180-120-30 = f. The 120 comes from d and the 30 comes from e. This leaves f as 30.
