Credit Card Payoff Planner implementation

Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

Record owner

Guth Instruments

Lifecycle

verified-clean

Evidence ID

t4-056

Effect

Computation or source lookup only

Input and validation requirements

The literal logarithmic payoff formula breaks at APR = 0 through a 0/0 expression, and the under-minimum-payment branch produces no finite payoff. The specification also leaves APR convention ambiguous, allowing materially different results.

Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

{
  "inputs": {
    "balance": 1000,
    "apr": 0.12,
    "monthly_payment": 100
  },
  "expected_outputs": {
    "analytical_months_to_payoff": 10.5886,
    "scheduled_months_to_payoff": 11,
    "total_interest": 58.9849,
    "min_payment_trap_warning": false
  },
  "working": "Monthly rate: r = 0.12/12 = 0.01. Interest accrued each month initially: 1000 x 0.01 = 10, so P = 100 > 10. Analytical months: n = ln(100/(100 - 0.01 x 1000)) / ln(1.01) = ln(100/90) / ln(1.01) = 0.1053605/0.00995033 = 10.5886. After 10 full payments: B10 = 1000(1.01)^10 - 100[((1.01)^10 - 1)/0.01] = 58.4009. Final payment after one more month of interest: 58.4009 x 1.01 = 58.9849. Total paid = 10 x 100 + 58.9849 = 1058.9849. Total interest = 1058.9849 - 1000 = 58.9849."
}

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