This problem draws on angle properties of polygons and factors of
360. Students need to work systematically to find solutions and
then reason carefully to justify the completeness of their
There is great value in comparing the combinations that work for
81, for 27 and for 54, and then accounting for any observed
connections between the relative sizes of angles and number of
Some rich directions in which to open up the task might include:
Is there an infinite number of combinations that will make 81,
or for that matter, any specified angle? If not, can we know how
many there will be?
It is to be hoped that this kind of questioning allows students to
reflect on the extent to which the unit of angle is arbitary (the
degree as one part in 360 for a complete rotation). Although 81, or
another number, has no decimal part, the two angles that together
make that sum may have decimal parts, or at least that possibility
needs considering . . .