If r={(x, y)|x^2+y^2=a^2} is a relation then its inverse r'={(x, y)|y^2+x^2=a^2} would be same as r, why is that, can someone please explain it with detail.

The oates family went out for dinner the price of the meal was $33.95 the sales tax was 7.25% of the price of the meal . the tip was 15% of the meal and the sales tax . how much money did the oates family pay for the meal, including tax and tip