Teller Window

Percent, earned interest, leftover balance — then a check, a sale, or a car lot.

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Offline copy — no account, no network required

Updated 2026-09-06

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Grades 6–8 · bank math

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Progress is stored only in localStorage on this device. No data is sent anywhere. No bank brands, no accounts.

Level 1

1 / 8

Percent of a number

What is 10% of $80?

Teller tip: Percent means hundredths. 10% is 10 per 100, or one tenth.

Keyboard: type the amount, Enter checks. L opens the Ledger Lab.

How Teller Window works

1. Percent
Find a percent of a dollar amount. 10% of $80 is one tenth of 80.

2. Simple interest
Interest = principal × rate × years. Then add it for the new balance.

3. Interest on interest
Read the year table. Year 2 uses last year’s ending balance, not the start.

4. Minimum due
Cards add interest each month (APR ÷ 12) on what you still owe. Paying more than the minimum leaves less to grow.

5. Put it together
A restaurant check, a sale plus tax, or cash vs a car loan. Monthly payments are printed — you find the total.

Type 20, $20, or 20.00. No calculator needed — the numbers are chosen that way. Ledger Lab lets you drag a payment and watch months and interest.

Ledger Lab

$800 card at 18% APR. Monthly rate is 18% ÷ 12 = 1.5%. Drag this month’s payment — the rest of the slip stays put.

Payment this month

$40Payment

—Months to pay off

—Total interest

Slip stamped

Teller Window

Want to keep this? Take a screenshot — Windows Win+Shift+S · Mac Shift+Command+4 · Chromebook Ctrl+Show windows.