19.40 On a plane, there are six points $A, B, C, D, E, F$ forming the shape shown in the figure (a polygon with folds). Then $\angle A + \angle B + \angle C + \angle D + \angle E + \angle F$ equals
(A) $180^{\circ}$.
(B) $360^{\circ}$.
(C) $540^{\circ}$.
(D) None of the above.
(Shanghai Junior High School Mathematics Competition, 1983)
[Solution] The sum of these angles is equivalent to the sum of the interior angles of three triangles minus the three interior angles of one triangle.
That is, the required angle sum is $3 \cdot 180^{\circ}-180^{\circ}=360^{\circ}$. Therefore, the answer is $(B)$.