There are 21 animals in the barn. some ducks and some are hires. there are 76 legs in all. how many of each animal are there?

Suppose you are a freshmen in college and you have not declared a major. one reason you enrolled at your respective college was the school's vibrant social life. at the same time, you want to balance your academic life because you know that your grades will matter for graduate school or job applications.you decide to forgo partying for two hours to study for an upcoming economics exam. in your first semester's principles of economics class, you learn that the enjoyment you would have received from partying is an example of a(n)