Translate a real-world linear situation into slope-intercept form and use it to answer questions in both directions.
Every one of these stories hides two numbers: a starting (or flat) amount -- the value even with zero of the changing thing -- and a rate, the amount added for every additional unit. Words like "per," "each," and "every" signal the rate; a flat fee or starting value signals b.
Once you have y = mx + b, use it two ways: plug in x to predict y (forward), or plug in y and solve for x (backward) -- the exact same equation-solving skill from the first two weeks, now attached to a real situation.
Mastery looks like: Given a real-world description, they correctly identify m and b, write y = mx + b, and use it to predict in either direction.
Common stumbles: Swapping which number is m and which is b; forgetting to label x and y, which leads to plugging numbers into the wrong spot later.