# Skill variation ladder
Skill: [prealg.double-number-line.missing-value](/math/skills/prealg/double-number-line-missing-value)
Family: Ratio, Rate, Scaling, and Proportional Readiness
Double number line missing values variation ladder grows by changing representation, numeric range, constraint count, distractors, language load, and transfer context.
## Variations
- 6. Advanced enrichment: Reason about the general method or a non-routine variant.
- 5. Challenge extension: Combine the skill with a second idea, hidden constraint, or explanation requirement.
- 3. Core task: Solve the target skill independently.
- 1. Foundation recognition: Notice the relevant quantity, shape, relation, or constraint before calculating.
- 2. Guided strategy: Use a visible scaffold, model, or narrowing move to choose a valid next step.
- 4. Transfer form: Apply the same idea when numbers, wording, or representation changes.
## Hints and Checks
- Decision restated without adding a new method.
- Common slip nudge: Uses the same additive jump where a multiplicative relationship is required.
- One useful scaffold after help is requested.