A book says someone paid 200 toward a loan. It does not follow that the amount owed fell by 200. To understand the example, find out how the payment was allocated before subtracting anything from the balance.
Separate the payment’s components
Principal is the borrowed amount still being repaid; interest and other applicable charges are separate components. In its explanation of mortgage repayment, the CFPB shows why a payment can include principal and interest, with additional amounts for items such as taxes and insurance. That mortgage example should not be copied as the structure of every loan.
Work through a deliberately small example
Suppose a fictional statement allocates a payment of 200 as 150 to principal and 50 to interest, with no other amounts in this simplified case. The principal decreases by 150, not 200. If the opening principal was 1,000, the resulting principal is 850 under those stated assumptions.
The arithmetic does not establish an interest rate or a repayment schedule. Those would require additional information about timing and the agreement. Keep the simplification beside the numbers.
Read the actual labels before using the idea
For a real account, use the lender’s statement and agreement to identify the allocation, fees and timing. Ask about unexplained components rather than assuming that a larger payment always produces the same balance change.
This builds on keeping assumptions beside a financial example. When reading, underline “payment” and “principal reduction” in different colours. They may be related numbers, but treating them as synonyms can make an otherwise neat calculation misleading.