A researcher wants to assess the difficulty of test questions in an algebra class at a high school. To test it, he takes one question from each test, each week in the first semester. In statistical language, each test question is part of the:

Cherie works in retail and her weekly salary includes commission for the amount she sells. the equation s=400+0.15c models the relation between her weekly salary, s, in dollars and the amount of her sales, c, in dollars. interpret the slope of the equation.