If this is homework, and the problem to be solved is only the problem stated, then my comment should be ignored, and I apologize for preaching.

If, however, this calculator is to be used in the real world, you have made incorrect assumptions: a customer may place a deposit of any amount, not just the discrete 5%, 10%, 15% and 20% examples. Further, 'auto loan' ignores student loans, other mortgages, levies, revolving debt, alimony, child support and many other forms of debt, so the field should be renamed at a minimum. Setting interest as a single value ignores the many varied instruments that make up the spectrum of loan types.