Angles on a line, at a point & vertically opposite

This whole lesson rests on three facts, and almost every mark lost in it comes from using the wrong one. Angles that sit side by side on a STRAIGHT LINE add up to 180°, because a straight line is a half turn. Angles that go all the way round a POINT add up to 360°, because that is a full turn. And when two straight lines cross, the angles directly ACROSS the crossing point from each other — the vertically opposite pair — are equal, with nothing to work out at all. So the first thing to do is never to start calculating. Look at the picture and ask: do these rays stop on a straight line, or do they carry on all the way round? Then subtract from 180 or from 360 to match. One warning that applies to every question here: PSLE figures are not drawn to scale, so an angle that looks like a right angle is only a right angle if the question says so.