Suppose that the function f (x) = 4x + 15 represents the cost to rent x video games a month from an internet gaming club. George now has $3. How many more dollars does George need to rent 8 video games next month?

A problem with one variable: Al's father is 45. He is 15 years older than twice Al's age. How old is Al? But what if we can't express all the information in the problem using only one variable? Sometimes we must use two variables to express the given information, which means that we need two equations to solve for these variables.