Write an equation from a rule

Target: Write y = mx + b from a per-unit rate and a starting amount.

Language target: Say which number is “per” and which is the start.

Likely miss: Students write the start-up cost as the coefficient.

Ask: What is the cost when the variable is zero?

If you see it: The per-unit amount is the coefficient. The starting amount is the constant.

Evidence to listen for: The equation matches a “for each” sentence.

Next: Test the equation at zero groups and at one group.

Observable student moves: names both quantities, uses the represented relationship, and revises after feedback. This companion is not a score.