Use the property of the exterior angle of a triangle

Follow this one with me, a step at a time. Try to guess each step first.

Worked example 1

In △ABC, BC is produced to D, ∠ACD = 140° and ∠A = 93°. Find ∠B in degrees.

Remember: ∠ACD is an exterior angle of the triangle. It equals the sum of the two interior opposite angles, ∠B and ∠A.

∠ACD is an exterior angle of △ABC, so ∠ACD = ∠A + ∠B.
140° = 93° + ∠B.
So ∠B = 140° − 93° = 47°, and the answer is 47.

Common mistakes

Used only one interior opposite angle. The exterior angle equals the sum of both.
Gave the interior angle at that vertex, not the exterior angle.
Subtracted from 360°. An exterior angle and its interior angle lie on a straight line, so together they make 180°.