The annual rate your bank charges you to borrow money for 1, 5, 10, or 30 years is shown in the following table.

If you borrow money from the bank for any duration from 1–30 years that’s not listed in the table, your rate is found by interpolating the appropriate number between the rates given in the table. For example, say you borrow money for 15 years. Because 15 years is one-quarter of the way between 10 years and 30 years, the annual loan rate would be calculated as follows: .75*.09 + .25*.10. Write a formula that returns the annual interest rate on a loan for any period between 1 and 30 years.

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Duration of loan 1 year 5 years 10 years 30 years Annual loan rate 6% 7% 9% 10%