The so-called "dense shop" can just spell out a complete 360.

Exceeding or not enough is unacceptable.

Then just find out the degree of the internal angle of the regular triangle, the regular quadrilateral ... a regular decagon.

Regular triangle: 60, right.

Regular quadrilateral: 90, right.

Regular Pentagon: 108, no.

Regular hexagon: 120, yes.

Regular heptagon: 900/7, no.

Regular octagon: 135, no.

True nonagon: 140, No.

Regular decagon: 144, no.