mel's photography borrowed $15000 on march 10 on demand note .the loan was repaid by payments of $4000 on june 20,$3000 on september 1 , and the balance on november 15. interest calculated on the daily balance and charged to mel's photography current account on the last day of each month ,was at 5.5% on march 10 but was changed to 6.25% effective june 1 and 6% effective october 1 . how many did the loan cost?