The fan-shaped structure is based on a circle. The area ratio of a sector to a circle is the percentage of the value represented by the sector in the whole value.

A circle can be regarded as a sector of 360, so 72 or 54 is the angle of the sector.

Then the percentage is 72/360 times 100%=20% which is the final answer (or 54/360 times 100%= 15%).