142. What is the sum of the exterior angles at the four vertices of a convex quadrilateral?
$\Delta$ To find the sum of these angles (they are shown on the diagram with single arcs, but this does not mean they are equal!), consider the corresponding interior angles. Together with the 4 interior angles (which sum up to $360^{\circ}$), we get 4 straight angles, that is, $720^{\circ}$, so the sum of the exterior angles is $720-360=360^{\circ} . \triangleleft$