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IFDM-127: Fixes to the interest calculation #83

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IFDM-127: Fixes to the interest calculation #83

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d373cb0
IFDM-127: Fixes to the interest calculation
jenbreese Mar 27, 2026
b4780d2
Merge branch '1.x' into IFDM-127
jenbreese Mar 27, 2026
48695ee
Merge branch '1.x' into IFDM-127
imonroe Mar 31, 2026
7723acf
Removed the max persiod
jenbreese Mar 31, 2026
8c93186
Merge branch 'IFDM-127' of github.com:SU-SWS/ifdm_learning_apps into …
jenbreese Mar 31, 2026
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57 changes: 26 additions & 31 deletions app/interactives/interest-calculator/page.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -10,45 +10,44 @@ const InterestRateVisual = () => {
const [mode, setMode] = useState("saving"); // 'saving' or 'borrowing'
const [amount, setAmount] = useState<number>(100);
const [interestRate, setInterestRate] = useState<number>(5);
const [years, setYears] = useState<number>(10);
const [periods, setPeriods] = useState<number>(10);
const [compounding, setCompounding] = useState("annually");

const [interestAmount, setInterestAmount] = useState(0);
const [totalAmount, setTotalAmount] = useState(0);
const MAX_YEARS = 240;

// Calculate the interest and total amount
useEffect(() => {
let periods = 1;
let periodsPerYear = 1;
switch (compounding) {
case "daily":
periods = 365;
periodsPerYear = 365;
break;
case "weekly":
periods = 52;
periodsPerYear = 52;
break;
case "bi-weekly":
periods = 26;
periodsPerYear = 26;
break;
case "monthly":
periods = 12;
periodsPerYear = 12;
break;
case "quarterly":
periods = 4;
periodsPerYear = 4;
break;
case "semi-annually":
periods = 2;
periodsPerYear = 2;
break;
default:
periods = 1;
periodsPerYear = 1;
}

// Compound interest formula: A = P(1 + r/n)^(nt)
// Compound interest formula: A = P(1 + r/n)^(t)
// where t is the number of compounding periods
const rate = interestRate / 100;
const periodicRate = rate / periods;
const totalPeriods = periods * years;
const periodicRate = rate / periodsPerYear;

const calculatedTotal = amount * Math.pow(1 + periodicRate, totalPeriods);
const calculatedTotal = amount * Math.pow(1 + periodicRate, periods);
const calculatedInterest = calculatedTotal - amount;

setInterestAmount(
Expand All @@ -57,7 +56,7 @@ const InterestRateVisual = () => {
setTotalAmount(
mode === "saving" ? calculatedTotal : amount + calculatedInterest
);
}, [amount, interestRate, years, compounding, mode]);
}, [amount, interestRate, periods, compounding, mode]);

return (
<div className="p-6 max-w-5xl mx-auto">
Expand Down Expand Up @@ -227,12 +226,11 @@ const InterestRateVisual = () => {
<input
type="number"
min="1"
max={MAX_YEARS.toString()}
placeholder="Enter years"
value={years === 0 ? "" : years}
placeholder="Enter periods"
value={periods === 0 ? "" : periods}
onChange={(e) =>
setYears(
Math.min(MAX_YEARS, Math.max(0, parseInt(e.target.value) || 0))
setPeriods(
Math.max(0, parseInt(e.target.value) || 0)
)
}
className="block w-full rounded-md shadow-sm py-2 px-3 border pr-10 [appearance:textfield] [&::-webkit-outer-spin-button]:appearance-none [&::-webkit-inner-spin-button]:appearance-none"
Expand All @@ -241,23 +239,20 @@ const InterestRateVisual = () => {
<button
type="button"
tabIndex={-1}
aria-label="Increase years"
onClick={() => setYears((prev) => Math.min(MAX_YEARS, prev + 1))}
disabled={years >= MAX_YEARS}
className={`mb-[-5px] hover:text-grey-med-dark focus:outline-none ${
years >= MAX_YEARS ? 'opacity-30 cursor-not-allowed' : ''
}`}
aria-label="Increase periods"
onClick={() => setPeriods((prev) => Math.min(prev + 1))}
className="mb-[-5px] hover:text-grey-med-dark focus:outline-none"
>
<BiSolidUpArrow size={24} />
</button>
<button
type="button"
tabIndex={-1}
aria-label="Decrease years"
onClick={() => setYears((prev) => Math.max(1, prev - 1))}
disabled={years <= 1}
aria-label="Decrease periods"
onClick={() => setPeriods((prev) => Math.max(1, prev - 1))}
disabled={periods <= 1}
className={`hover:text-grey-med-dark focus:outline-none ${
years <= 1 ? 'opacity-30 cursor-not-allowed' : ''
periods <= 1 ? 'opacity-30 cursor-not-allowed' : ''
}`}
>
<BiSolidDownArrow size={24} />
Expand Down Expand Up @@ -338,7 +333,7 @@ const InterestRateVisual = () => {
maximumFractionDigits: 2,
})}
</div>
</div>
</div>
</div>
{/* Example section */}
<div className="mt-6 py-4 align-self-top lg:mt-0 lg:py-0">
Expand Down