This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
That is not simple interest you are calculating, but compound interest. If you find the most dubious classification of methods known to modern science, that comes to be a 1368. You could therefore consider making it static.
Note you can’t override static methods, nor make interface methods static.