If you have €100,000 and want to calculate how much interest you would earn at an interest rate of 4% per year, you can use a simple formula.The formula for calculating the annual interest rate is:Interest = Principal amount × Interest ratePrincipal amount = €100,000 Interest rate = 4% (or 0.04 as a decimal)The annual interest rate is:€100,000 × 0.04 = €4,000This means that if you save €100,000 in an account with an interest rate of 4%, you would earn €4,000 interest per year.