Build the Calculation Engine

public class LoanCalculationService {
    public Decimal calculateInterestRate(Decimal creditScore) {
        if (creditScore >= 700) {
            return 6.5;
        } else if (creditScore >= 600) {
            return 9.0;
        } else {
            return 14.0;
        }
    }

    public Decimal calculateMonthlyPayment(Decimal amount, Decimal annualRatePercent, Integer months) {
        Decimal monthlyRate = (annualRatePercent / 100) / 12;
        return (amount * monthlyRate) / (1 - Math.pow(1 + monthlyRate, -months));
    }
}

calculateInterestRate is Module 6's if/else if/else chain; calculateMonthlyPayment implements the standard amortized-loan-payment formula, taking the principal, annual rate, and loan term in months.

@isTest
private class LoanCalculationServiceTest {
    @isTest
    static void highCreditScoreGetsLowestRate() {
        LoanCalculationService service = new LoanCalculationService();
        System.assertEquals(6.5, service.calculateInterestRate(750));
    }

    @isTest
    static void midCreditScoreGetsMiddleRate() {
        LoanCalculationService service = new LoanCalculationService();
        System.assertEquals(9.0, service.calculateInterestRate(650));
    }

    @isTest
    static void lowCreditScoreGetsHighestRate() {
        LoanCalculationService service = new LoanCalculationService();
        System.assertEquals(14.0, service.calculateInterestRate(550));
    }
}

Three test methods, one per branch of the if/else if/else chain — this is Module 31's Lesson 2 habit ("one test method, one scenario") applied the moment the logic is written, not saved for a separate lesson afterward.

@isTest
static void creditScoreOf700IsTheLowestQualifyingForBestRate() {
    LoanCalculationService service = new LoanCalculationService();
    System.assertEquals(6.5, service.calculateInterestRate(700));
}

@isTest
static void creditScoreOf699JustMissesTheBestRate() {
    LoanCalculationService service = new LoanCalculationService();
    System.assertEquals(9.0, service.calculateInterestRate(699));
}

Testing exactly 700 and 699 — the boundary between two branches — is where off-by-one bugs (> instead of >=, from Module 4) actually hide. This is Module 31's "assertions that actually tell you something" lesson, applied specifically to boundary conditions.