Chapter 5 - Number Theory and the Real Number System - 5.5 Real Numbers and Their Properties; Clock Addition - Exercise Set 5.5 - Page 310: 77

Seven-fold rotational symmetry.

Consider the given design. Rotate it with the${{\left( \frac{360}{7} \right)}^{\circ }}={{51.43}^{\circ }}$. It will be same as original shape. On $7$ such rotations, the same position will achieve. Thus, the given design has seven-fold symmetry.