Devon found that for every 4 miles he walks, he needs one bottle of water. He plotted his water usage on a graph but left out some possible points. On a coordinate plane, the points (4, 1) and (12, 3) are plotted.

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.