Apply systems of equations to a break-even comparison project -- set up two linear cost models and solve for the point where they are equal.
Break-even is the point where two plans cost exactly the same -- a system-of-equations question wearing a business costume. Build two equations (each with a starting amount and a rate), set them equal, solve, and interpret what happens on each side of that point.
Building each plan's equation correctly, separately, matters more than anything else -- a wrong single equation ruins the whole comparison.
Mastery looks like: Given two real-world plans, they build both equations, solve for the break-even point, and correctly state which plan wins on each side of it.
Common stumbles: Building one equation with the rate and starting amount reversed; finding the break-even x-value but forgetting to also check which plan wins elsewhere.