Principal: $13,500, 6t, 240 days Partial payments Given On 100th day, $5,200 On 180th day, $3,100 a. Use the U.S. Rule to solve for total interest cost. (Use 360 days a year. Do not round intermediate calculations. Round your answer to the nearest cent.) Total interest cost b. Use the U.S. Rule to solve for principal balances. (Use 360 days a year. Do not round intermediate calculations. Round your answers to the nearest cent.) On 100th day On 180th day Balance after the payment c. Use the U.S. Rule to solve for final payment. (Use 360 days a year. Do not round intermediate calculations. Round your answer to the nearest cent.)