Sir Cumference and the Great Knight of Angleland — Double the Right Angle — Teacher Key
The three numbers
The traced opening measures 180 degrees. The two the student started with are 90 degrees and 90 degrees. The sentence the step asks for is ninety and ninety make one hundred eighty.
What the two outer edges look like
They lie in one straight line. Expect straight, flat, a straight line, or one long line rather than a technical word.
The argument with the medallion covered
There is no single wording. A strong argument uses both corners and both numbers: each corner is 90 degrees, they are joined so no gap and no overlap is left between them, so together they cover 90 and 90, which is 180 degrees, and 180 degrees is the straight angle. The partner's one question must be answered with a number in it, so a strong answer names 90, 180, or the sum.
The last claim
Folding the traced straight angle in half along the dot gives two halves, and one half measures 90 degrees, a right angle. A strong answer says yes and points to the fold that made the two halves match, then gives the number the medallion read.
Read for
A close argument would still hold if the corners were a different size on the page, because it rests on the two 90s and on the join leaving no gap. A loose argument reports the measurement again (I measured it and it said 180) or appeals to the look of it (it just goes flat), which is the thing covering the medallion was meant to take away. Support says the same argument from the frame with the corners in hand; that is the target, not a lesser one.
Extension
Three right angles joined the same way make 270 degrees, which is more than a straight angle and not a straight line. Four right angles close the circle at 360 degrees; expect students to lay all four corners round the dot before they say the number.