Give an example of a matrix which is its own inverse (that is, where a^-1 = a)

Why did the ice arena get so hot after the big game (this is math related google it to find the paper

Two farmers pay a total rent of $140 for a pasture. the first farmer placed $4 oxen in the pasture, and the second farmer placed $30 sheep in the pasture. the farmers split the rent based on the proportion of the pasture eaten by their own animals that month. how much should the first farmer pay, if 1 ox eats as much as 10 sheep in a month?

Which phrase generalizes the specific terminology bolded in the text?