The three interior angles of a triangle are in the ratio 2 : 3 : 4. What is the largest of its three exterior angles?

The angles are 2k + 3k + 4k = 180°, so k = 20° and the interior angles are 40°, 60° and 80°. Each exterior angle is the supplement of its interior angle, giving 140°, 120° and 100°. The largest exterior angle sits at the smallest interior angle, so it is 140°. Choosing 100° is the trap of pairing the largest exterior angle with the largest interior angle.