Could you please explain number 7 on page 29?

This is in the plug in chapter because you can get it simply by making up numbers that add up to 180° inside the triangle. Say, for example, that the angles inside the triangle are 50°, 60°, and 70°. Now use supplemental angle rules to fill in the exterior angles of the triangle. For example, the 50° angle will have 130° angles on either side of it.

Now add them all up and you’ll get 900°.

If you want to know *why* that works, note that a full circle is 360°, and each of the marked angles is almost a full circle, but missing one piece. And it turns out, the missing pieces are all vertical angles of the angles inside the triangle. Since you know a triangle has 180°, you know that the sum of all the marked angles is 3(360°)-180°.