Every city has several attractions. A city has a name and a specific number of inhabitants, while an attraction has a name and an address. Various guided tours are offered for a tourist attraction, and the duration and number of a guided tour are known. A maximum of 20 visitors can take part in a guided tour, and certain visitors may be granted a discount for certain guided tours; the amount of this discount must be known. Of course, a guided tour also requires a guide. The visitors and the guide are people who have names.