how to do this math?

Transaction Amount Transaction Description Previous balance: $3150.00 March 1 Billing date March 4 Payment March 6 Charge: Furniture March 15 Charge: Medical March 21 Charge: Books March 29 Charge: Groceries March 31 End of billing period Payment Due Date: April 1 $ 400.00 credit $ 1200.00 $ 150.00 $ 82.50 $ 145.00 Date Unpaid Balance Number of Days Unpaid Balance at each unpaid Number of days balance