The statement that is true of interest is: "It is a percentage of the amount of the principal you have not yet paid back."
Interest is typically calculated on the remaining balance of the principal for loans, which means that the interest amount can change over time as you make payments and reduce the principal balance.
The other statements are not accurate in the context of how interest typically works:
- "It is a percentage of the total cost of the loan" does not correctly capture how interest is calculated.
- "It is calculated only once, when you first take out the loan" is incorrect because interest is generally calculated on an ongoing basis.
- "It is expressed as the APR of a loan" is not entirely accurate; APR (Annual Percentage Rate) is a measure of the cost of borrowing expressed as a yearly interest rate, but it is not the same as interest itself.