Road Trip Cost Splitter mechanism

Model scope

No arithmetic formula is inherently incorrect, but the instrument is not fully specified for production use. The exact ambiguity requiring review is whether 'optional per-leg mileage' means mileage per passenger, mileage per leg, or passenger-by-leg mileage, and how those values are aggregated. The honest label should state that this is a user-selected cost-allocation calculator, not an objective determination of fairness; mileage weighting is specifically proportional-distance allocation.

• The equal-split formula is correct only when every listed person shares the cost equally: per_person = total_cost / num_people.
• The mileage-weighted formula is mathematically correct for allocating cost in proportion to distance ridden, but total_miles must be explicitly defined as the sum of all passengers' distances.
• The specification says 'optional per-leg mileage' but does not define whether mileage is supplied per passenger, per leg, or both, nor how a passenger's total mileage is calculated across legs.
• The specification does not define how the user selects equal splitting versus mileage-weighted splitting.
• The word 'fairly' is ambiguous: mileage weighting may not be fair for tolls, shared fixed costs, drivers, passengers joining or leaving mid-trip, or different vehicle occupancy by leg.

Units and conventions

• total_cost is a nonnegative monetary amount in one currency, and all results use that same currency.
• num_people is a positive integer.
• For mileage weighting, every person's mileage uses the same distance unit, such as miles or kilometers; the unit cancels in the ratio only when used consistently.
• For mileage weighting, total_miles equals the sum of all listed passengers' miles and must be greater than zero.
• If costs are entered in cents rather than currency units, the output is also in cents unless an explicit conversion is applied.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "mode": "equal",
    "total_cost": 120,
    "currency": "USD",
    "num_people": 4
  },
  "expected_outputs": {
    "amount_owed_per_person": [
      30,
      30,
      30,
      30
    ]
  },
  "working": "per_person = total_cost / num_people = 120 / 4 = 30. Each of the 4 passengers owes $30."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

The equal-split formula is undefined when num_people is zero. The mileage-weighted formula also requires a nonzero total_miles; zero total mileage would create another undefined division.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche4/tranche4-results.json
• BOUNDARY_HAZARD_CATALOG.json