Watch this short video on rotational symmetry.  

It explains something called the order of rotational symmetry.

The order of symmetry is the number used to describe how many times a diagram will appear to be the same while rotating through 360°.

If the diagram matches up twice, it is Order 2.

If it matches the original shape three times, it is Order 3.

Order 2 = 180°

Order 4 = 90°

Order 8 = 45°

Example

In the following diagram, what colour must go in square A for the shape to have Rotational Symmetry of Order 2.

Answer

A must be BLUE